In a healthy adult human, about 500 mg of cholesterol is converted to bile salts daily. Newly synthesized bile salts are secreted into the bile and released into the small intestine where they emulsify dietary fats (Russell 2003). About 95% of the bile salts in the intestine are recovered and returned to the liver (Kullak-Ublick et al. 2004; Trauner and Boyer 2002). The major pathway for bile salt synthesis in the liver begins with the conversion of cholesterol to 7alpha-hydroxycholesterol. Bile salt synthesis can also begin with the synthesis of an oxysterol - 24-hydroxycholesterol or 27-hydroxycholesterol. In the body, the initial steps of these two pathways occur in extrahepatic tissues, generating intermediates that are transported to the liver and converted to bile salts via the 7alpha-hydroxycholesterol pathway. These extrahepatic pathways contribute little to the total synthesis of bile salts, but are thought to play important roles in extrahepatic cholesterol homeostasis (Javitt 2002).
View original pathway at Reactome.
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This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
Cholate or chenodeoxycholate, coenzyme A, and ATP react to form their CoA conjugates, AMP, pyrophosphate and water. This reaction is catalyzed by SLC27A5 (BACS) associated with the endoplasmic reticulum membrane, but the substrates and products are cytosolic (Mihalik et al. 2002).
Cytosolic bile acid-CoA conjugates (choloyl-CoA; chenodeoxycholoyl-CoA) react with the amino acids glycine and taurine, generating the corresponding bile salts and coenzyme A, catalyzed by BAAT (bile acid-CoA:amino acid N-acetyltransferase). In the body, this reaction occurs in hepatocytes and is the means by which bile acids recovered from the intestine are converted to bile salts before being released again into the bile (Kullak-Ublick et al. 2004; Trauner and Boyer 2002).
THCA (3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoate) is translocated from the mitochondrial matrix to the cytosol. The transporter that mediates its passage across the inner mitochondrial membrane has not been identified. In particular, despite the structural and functional similarities between THCA and long chain fatty acids, searches for carnitine - THCA have failed (Russell 2003). VLCS (SLC27A2), one of the enzymes that catalyzes CoA conjugation of THCA may also be present in peroxisomes, and Mihalik et al. (2002) have hypothesized that THCA could be translocated unchanged from the mitochondrial matrix to the peroxisomal matrix and undergo conjugation there.
27-hydroxycholesterol, NADPH + H+, and O2 react to form cholest-5-ene-3beta,7alpha,27-triol, H2O, and NADP+. This reaction is catalyzed by CYP7B1 in the endoplasmic reticulum membrane. Defects in CYB7B1 are associated with failure of 7alpha-hydroxylation in vivo, and with liver damage, confirming both the function of the enzyme and the central role of the liver in this metabolic process (Setchell et al. 1998).
The microsomal enzyme cholesterol 25-hydroxylase is a member of a lipid metabolizing enzyme family that utilizes oxygen and diiron-oxygen cofactor to hydroxylate, desaturate, epoxidate and acetylate substrates.
CYP27A1, a mitochondrial matrix sterol hydroxylase, catalyses the 27-hydroxylation of side-chains of sterol intermediates (Cali et al. 1991). A specific reaction is the 27-hydroxylation of 5beta-cholestan-3alpha, 7alpha, 12alpha-triol (5bCHOL3a,7a,12a-triol) to form 5beta-cholestan-3alpha, 7alpha, 12alpha, 27-tetrol (5bCHOL3a,7a,12a, 27-tetrol) (Pikuleva et al. 1998). Defects in CYP27A1 can cause Cerebrotendinous xanthomatosis (CTX; MIM:213700), a rare sterol storage disorder characterised by progressive neurologic dysfunction, premature atherosclerosis and cataracts (Cali et al. 1991b).
5beta-cholestan-3alpha,7alpha,12alpha-triol is translocated from the cytosol to the mitochondrial matrix. The transporter that mediates its passage across the inner mitochondrial membrane is unknown: the StAR protein that performs this function for cholesterol at the start of steroid hormone biosynthesis is excluded as StAR is not expressed in liver. Other members of the START family of transporters are candidates, however (Russell 2003).
4-Cholesten-7alpha-ol-3-one, NADPH, and H+ react to form 5beta-cholestan-7alpha-ol-3-one and NADP+. This reaction is catalyzed by AKR1D1 (3-oxo-5-beta-steroid 4-dehydrogenase). AKR1D1 is localized to the cytosol, and in the course of the reaction its steroid substrate moves from the endoplasmic reticulum membrane to the cytosol. It is unclear whether this translocation results simply from its increased hydrophilicity or is mediated by the enzyme or another transport protein (Russell 2003).
5Beta-cholesten-7alpha,12alpha-diol-3-one and NADPH + H+ form 5beta-cholestan-3alpha,7alpha,12alpha-triol and NAPDP+. The reaction is catalyzed by 3alpha-hydroxysteroid dehydrogenase (AKR1C4), a cytosolic enzyme belonging to the aldo-keto reductase family (Dufort et al. 2001). Biochemical studies with rat proteins raise the possibility that other related enzymes may also carry out this reaction in vivo (Russell 2003).
5beta-cholestan-3alpha, 7alpha, 12alpha, 27-tetrol and NADP+ react to form 3alpha, 7alpha, 12alpha-trihydroxy-5beta-cholestan-27-al and NADPH + H+. This oxidation reaction occurs in the mitochondrial matrix, catalyzed by CYP27A1.
Cholesterol, NADPH + H+, and O2 form 7alpha-cholesterol (5-cholesten-3beta, 7alpha-diol), NADP+,and H2O, in a reaction catalysed by CYP7A1 (cholesterol 7alpha-hydroylase) in the endoplasmic reticulum membrane. In the body, this enzyme is expressed only in liver, and its expression is tightly regulated at the level of transcription to determine the overall rate of bile acid and bile salt production (Noshiro et al. 1990).
3alpha,7alpha,12alpha-trihydroxy-5beta-cholestan-27-al, NADPH + H+, and O2 react to form 3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoate (THCA), NADP+, and H2O. This oxidation reaction occurs in the mitochondrial matrix, catalyzed by CYP27A1.
Cholesterol 24-hydroxylase (CYP46A1), an enzyme associated with the ER membrane, catalyses the 24-hydroxylation of cholesterol (CHOL) to 24-hydroxycholesterol (24OH-CHOL). In the body, this enzyme is expressed predominantly in the brain and is thought to play a major role in cholesterol turnover there (Lund et al. 1999, Mast et al. 2003).
25-hydroxycholesterol (25OH-CHOL) is 7alpha-hydroxylated to cholest-5-ene-3beta,7alpha,25-triol (CHOL3b,7a,25TRIOL) by CYP7B1 (cytochrome P450 7B1) (Wu et al. 1999).
4-Cholesten-7alpha, 12alpha-diol-3-one and NADPH + H+ react to form 5beta-cholesten-7alpha,12alpha-diol-3-one + NADP+. This reaction is catalyzed by AKR1D1 (3-oxo-5-beta-steroid 4-dehydrogenase). AKR1D1 is localized to the cytosol, and in the course of the reaction its steroid substrate moves from the endoplasmic reticulum membrane to the cytosol. It is unclear whether this translocation results simply from its increased hydrophilicity or is mediated by the enzyme or another transport protein (Russell 2003).
7alpha-hydroxycholesterol and NAD+ react to form 4-cholesten-7alpha-ol-3-one, NADH, and H+, in a reaction catalyzed by HSD3B7 (3 beta-hydroxysteroid dehydrogenase type 7) in the endoplasmic reticulum membrane. Its function in vivo has been confirmed in studies of patients with defects in bile acid synthesis (Schwarz et al. 2000).
Cholesterol CHOL), NADPH + H+, and O2 react to form 27-hydroxycholesterol (27OH-CHOL), H2O, and NADP+, in the mitochondrial matrix, catalysed by CYP27A1. 27OH-CHOL is the most abundant oxysterol in human plasma. Its formation is thought to play a central role in the mobilization of CHOL from non-hepatic tissues (Cali et al. 1991).
THCA (25(R) 3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoate) , coenzyme A, and ATP react to form the CoA conjugate of 25(R) THCA, AMP, and pyrophosphate. This cytosolic reaction is catalyzed by SLC27A5 (BACS). SLC27A2 (VLCS) also catalyzes this reaction; the relative contributions of the two enzymes to de novo bile acid synthesis in vivo are not certain (Mihalik et al. 2002).
Sterol 12alpha hydroxylase (CYP8B1), an enzyme associated with the endoplasmic reticulum membrane, catalyses the 12-alpha-hydroxylation of 7-Alpha-hydroxycholest-4-en-3-one (4CHOL7aOLONE) to 7-alpha,12-alpha-dihydroxycholest-4-en-3-one (4CHOL7a,12aDONE). While the human gene has been cloned (Gafvels et al. 1999), its protein product has not been characterised, and the enzymatic properties of human CYP8B1 protein are inferred from those of its well-characterised rabbit homolog (Ishida et al. 1992).
5Beta-cholesten-7alpha-ol-3-one and NADPH + H+ form 5beta-cholestan-3alpha,7alpha-diol and NAPDP+. The reaction is catalyzed by 3alpha-hydroxysteroid dehydrogenase (AKR1C4), a cytosolic enzyme belonging to the aldo-keto reductase family (Dufort et al. 2001). Biochemical studies with rat proteins raise the possibility that other related enzymes may also carry out this reaction in vivo (Russell 2003).
24-hydroxycholesterol 7-alpha-hydroxylase (CYP39A1), located on the ER membrane, 7-alpha-hydroxylates 24-hydroxycholesterol (24OH-CHOL) to cholest-5-ene-3beta,7alpha,24-triol (CHOL7a,24(S)DIOL). In the body, expression of CYP39A1 is restricted to the liver and is involved in bile acid metabolism (Li-Hawkins et al. 2000).
Choloyl CoA reacts with glycine or taurine to form glycocholate or taurocholate, releasing CoASH. This reaction, which completes the de novo synthesis of bile salts from cholesterol in vivo, is catalyzed by BAAT (Bile acid CoA:amino acid N-acyltransferase - Falany et al. 1994) and occurs in the peroxisomal matrix (Solaas et al. 2000; Mihalik et al. 2002). In vivo, the relative amounts of glycocholate and taurocholate synthesized appear to be determined solely by the intracellular abundances of glycine and taurine (Russell 2003).
The bile salts glycocholate, glycochenodeoxycholate, taurocholate, and taurochenodeoxycholate are translocated from the peroxisomal matrix to the cytosol. The transporter that mediates this process is unknown (Russell 2003).
25(R) THCA-CoA is transported from the cytosol into the peroxisome. Indirect evidence suggests that a member of the ABC class of transporters mediates this reaction.
3alpha,7alpha,12alpha-trihydroxy-5beta-cholest-24-enoyl-CoA (THCA-CoA) is hydrated to form (24R, 25R) 3alpha,7alpha,12alpha,24-tetrahydroxy-5beta-cholestanoyl-CoA. This reaction, catalyzed by the peroxisomal D-bifunctional enzyme (Huyghe et al. 2006), occurs in the peroxisomal matrix.
25(S) THCA-CoA and O2 react to form 3alpha,7alpha,12alpha-trihydroxy-5beta-cholest-24-enoyl-CoA (THCA-CoA) and H2O2. This dehydrogenation reaction occurs in the peroxisomal matrix. It is catalyzed by the FAD-containing peroxisomal enzyme branched chain acyl-CoA oxidase (ACOX2). The enzyme transfers electrons to molecular oxygen and hydrogen peroxide is produced as a byproduct.
3alpha,7alpha,12alpha-trihydroxy-5beta-cholan-24-one-CoA and CoASH react to form choloyl-CoA (3alpha,7alpha,12alpha-trihydroxy-5beta-cholan-24-one-CoA) and propionyl CoA. This reaction, in the peroxisomal matrix, is catalyzed by peroxisomal thiolase 2 (sterol carrier protein 2).
A molecule of glycocholate, glycochenodeoxycholate, taurocholate, or taurochenodeoxycholate is transported from the cytosol to the extracellular space, coupled to the hydrolysis of a molecule of cytosolic ATP to ADP and orthophosphate. This reaction is mediated by ABCB11 (bile salt export pump). In the body, this reaction mediates the release of bile salts from the liver cells into the bile (Kullak-Ublick et al. 2004); the role of ABCB11 in the reaction is confirmed by the observed failure of bile salt export in patients in whom the transporter is defective (Noe et al. 2005).
25(S) DHCA-CoA and O2 react to form 25(S) 3alpha,7alpha-dihydroxy-5beta-cholest-24-enoyl-CoA and H2O2. This dehydrogenation reaction occurs in the peroxisomal matrix. It is catalyzed by the FAD-containing peroxisomal enzyme branched chain acyl-CoA oxidase (ACOX2). The enzyme transfers electrons to molecular oxygen and hydrogen peroxide is produced as a byproduct.
The peroxisomal matrix enzyme acyl-coenzyme A thioesterase 8 (ACOT8) mediates the hydrolysis of choloyl-CoA to form cholate and CoASH (Jones et al. 1999, Hunt et al. 2002). While bile salts are the major product of the de novo biosynthetic pathway in the normal human liver, bile acids are major feedback regulators of this pathway and this hydrolysis reaction is thought to play a role in generating them (Russell 2003).
5beta-cholestan-3alpha, 7alpha-diol, NADPH + H+, and O2 react to form 5beta-cholestan-3alpha, 7alpha, 26-triol + NADP+ + H2O. This reaction occurs in the mitochondrial matrix, catalyzed by CYP27A1.
THCA (25(R) 3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoate), coenzyme A, and ATP react to form the CoA conjugate of 25(R) THCA, AMP, and pyrophosphate. This cytosolic reaction is catalyzed by SLC27A2 (VLCS). SLC27A5 (BACS) also catalyzes this reaction; the relative contributions of the two enzymes to de novo bile acid synthesis in vivo are not certain (Mihalik et al. 2002).
DHCA (25(R) 3alpha,7alpha-dihydroxy-5beta-cholestanoate) , coenzyme A, and ATP react to form the CoA conjugate of 25(R) DHCA, AMP, and pyrophosphate. This cytosolic reaction is catalyzed by SLC27A5 (BACS). SLC27A2 (VLCS) also catalyzes this reaction; the relative contributions of the two enzymes to de novo bile acid synthesis in vivo are not certain (Mihalik et al. 2002).
DHCA (25(R) 3alpha,7alpha-dihydroxy-5beta-cholestanoate), coenzyme A, and ATP react to form the CoA conjugate of 25(R) DHCA, AMP, and pyrophosphate. This cytosolic reaction is catalyzed by SLC27A2 (VLCS). SLC27A5 (BACS) also catalyzes this reaction; the relative contributions of the two enzymes to de novo bile acid synthesis in vivo are not certain (Mihalik et al. 2002).
(24R, 25R) 3alpha,7alpha,12alpha,24-tetrahydroxy-5beta-cholestanoyl-CoA and NAD+ react to form 3alpha,7alpha,12alpha-trihydroxy-5beta-cholest-24-one-CoA, NADH, and H+. This oxidation reaction, catalyzed by the peroxisomal D-bifunctional enzyme (Huyghe et al. 2006), occurs in the peroxisomal matrix.
3alpha, 7alpha-dihydroxy-5beta-cholestan-26-al, NADPH + H+, and O2 react to form 3alpha, 7alpha-dihydroxy-5beta-cholestanoate (DHCA), NADP+ and H2O. This oxidation reaction occurs in the mitochondrial matrix, catalyzed by CYP27A1.
25(R) DHCA-CoA is transported from the cytosol into the peroxisome. Indirect evidence suggests that a member of the ABC class of transporters mediates this reaction.
Chenodeoxycholoyl CoA reacts with glycine or taurine to form glycochenodeoxycholate or taurochenodeoxycholate, releasing CoASH. This reaction, which completes the de novo synthesis of bile salts from cholesterol in vivo, is catalyzed by BAAT (Bile acid CoA:amino acid N-acyltransferase - Falany et al. 1994) and occurs in the peroxisomal matrix (Solaas et al. 2000; Mihalik et al. 2002). In vivo, the relative amounts of glycochenodeoxycholate and taurochenodeoxycholate synthesized appear to be determined solely by the intracellular abundances of glycine and taurine (Russell 2003).
5beta-cholestan-3alpha, 7alpha, 26-triol and NADP+ react to form 3alpha, 7alpha-dihydroxy-5beta-cholestan-26-al and NADPH + H+. This oxidation reaction occurs in the mitochondrial matrix, catalyzed by CYP27A1.
(24R, 25R) 3alpha,7alpha,24-trihydroxy-5beta-cholestanoyl-CoA and NAD+ react to form 3alpha,7alpha-dihydroxy-5beta-cholest-24-one-CoA, NADH, and H+. This oxidation reaction, catalyzed by the peroxisomal D-bifunctional enzyme (Huyghe et al. 2006), occurs in the peroxisomal matrix.
DHCA (3alpha, 7alpha-dihydroxy-5beta-cholestanoate) is translocated from the mitochondrial matrix to the cytosol. The transporter that mediates its passage across the inner mitochondrial membrane has not been identified. In particular, despite the structural and functional similarities between DHCA and long chain fatty acids, searches for carnitine - DHCA have failed (Russell 2003). VLCS (SLC27A2), one of the enzymes that catalyzes CoA conjugation of DHCA, may also be present in peroxisomes, and Mihalik et al. (2002) have hypothesized that DHCA could be translocated unchanged from the mitochondrial matrix to the peroxisomal matrix and undergo conjugation there.
3alpha,7alpha-dihydroxy-5beta-cholan-24-one-CoA and CoASH react to form chenodeoxycholoyl-CoA (3alpha,7alpha-dihydroxy-5beta-cholan-24-one-CoA) and propionyl CoA. This reaction, in the peroxisomal matrix, is catalyzed by peroxisomal thiolase 2 (sterol carrier protein 2).
25(S) 3alpha,7alpha-dihydroxy-5beta-cholest-24-enoyl-CoA is hydrated to form (24R, 25R) 3alpha,7alpha,24-trihydroxy-5beta-cholestanoyl-CoA. This reaction, catalyzed by the peroxisomal D-bifunctional enzyme (Huyghe et al. 2006), occurs in the peroxisomal matrix.
5beta-cholestan-3alpha, 7alpha-diol is transported from the cytosol to the mitochondrial matrix. The transporter that mediates its passage across the inner mitochondrial membrane is unknown: the StAR protein that performs this function for cholesterol at the start of steroid hormone biosynthesis is excluded as StAR is not expressed in liver. Other members of the START family of transporters are candidates, however (Russell 2003).
Sterol 12alpha hydroxylase (CYP8B1) catalyses the 12-alpha-hydroxylation of 4-cholesten-7alpha,24(S)-diol-3-one (4CHOL7a,24(S)DONE) to 4-cholesten-7-alpha,12-alpha,24(S)-triol-3-one (4CHOL7a,12a,24(S)TONE). While the human gene has been cloned (Gafvels et al. 1999), its protein product has not been characterised, and the enzymatic properties of human CYP8B1 protein are inferred from those of its well-characterised rabbit homolog (Ishida et al. 1992).
3,7,24THCA (25(R) 3alpha,7alpha,24(S)-trihydroxy-5beta-cholestanoate), coenzyme A, and ATP react to form the CoA conjugate of 3,7,24THCA, AMP, pyrophosphate and water. This cytosolic reaction is catalyzed by SLC27A5 (BACS). SLC27A2 (VLCS) also catalyzes this reaction; the relative contributions of the two enzymes to de novo bile acid synthesis in vivo are not certain (Mihalik et al. 2002).
3alpha,7alpha,12alpha,24(S)-tetrahydroxy-5beta-cholestan-27-al, NADPH + H+, and O2 react to form 3alpha,7alpha,12alpha,24(S)-tetrahydroxy-5beta-cholestanoate (TetraHCA), NADP+, and H2O. This oxidation reaction occurs in the mitochondrial matrix, catalyzed by CYP27A1.
5beta-cholestan-3alpha,7alpha,24(S)-triol is transported from the cytosol to the mitochondrial matrix. The transporter that mediates its passage across the inner mitochondrial membrane is unknown: the StAR protein that performs this function for cholesterol at the start of steroid hormone biosynthesis is excluded as StAR is not expressed in liver. Other members of the START family of transporters are candidates, however (Russell 2003).
5beta-cholestan-3alpha,7alpha,24(S),27-tetrol, NADPH + H+, and O2 react to form 3alpha,7alpha,24(S)-trihydroxy-5beta-cholestan-27-al, NADP+, and H2O. This oxidation reaction occurs in the mitochondrial matrix, catalyzed by CYP27A1.
TetraHCA (3alpha,7alpha,12alpha,24(S)-tetrahydroxy-5beta-cholestanoate) is translocated from the mitochondrial matrix to the cytosol. The transporter that mediates its passage across the inner mitochondrial membrane has not been identified (Russell 2003). VLCS (SLC27A2), one of the enzymes that catalyzes CoA conjugation of TetraHCA, may also be present in peroxisomes, and Mihalik et al. (2002) have hypothesized that TetraHCA could be translocated unchanged from the mitochondrial matrix to the peroxisomal matrix and undergo conjugation there.
TetraHCA (25(R) 3alpha,7alpha,12alpha,24(S)-tetrahydroxy-5beta-cholestanoate), coenzyme A, and ATP react to form the CoA conjugate of 25(R) TetraHCA, AMP, pyrophosphate and water. This cytosolic reaction is catalyzed by SLC27A2 (VLCS). SLC27A5 (BACS) also catalyzes this reaction; the relative contributions of the two enzymes to de novo bile acid synthesis in vivo are not certain (Mihalik et al. 2002).
The isomerization of 3,7,24THCA-CoA to (24R, 25R) 3alpha,7alpha,24-trihydroxy-5beta-cholestanoyl-CoA, catalyzed by 2-methylacyl-CoA racemase, occurs in the peroxisomal matrix.
3alpha,7alpha,24(S)-trihydroxy-5beta-cholestan-27-al, NADPH + H+, and O2 react to form 3alpha,7alpha,24(S)-trihydroxy-5beta-cholestanoate (3,7,24THCA), NADP+, and H2O. This oxidation reaction occurs in the mitochondrial matrix, catalyzed by CYP27A1.
3,7,24THCA (25(R) 3alpha,7alpha,24(S)-trihydroxy-5beta-cholestanoate), coenzyme A, and ATP react to form the CoA conjugate of 3,7,24THCA, AMP, pyrophosphate and water. This cytosolic reaction is catalyzed by SLC27A2 (VLCS). SLC27A5 (BACS) also catalyzes this reaction; the relative contributions of the two enzymes to de novo bile acid synthesis in vivo are not certain (Mihalik et al. 2002).
4-Cholesten-7alpha,24(S)-diol-3-one, NADPH, and H+ react to form 5beta-cholestan-7alpha,24(S)-diol-3-one and NADP+. This reaction is catalyzed by AKR1D1 (3-oxo-5-beta-steroid 4-dehydrogenase). AKR1D1 is localized to the cytosol, and in the course of the reaction its steroid substrate moves from the endoplasmic reticulum membrane to the cytosol. It is unclear whether this translocation results simply from its increased hydrophilicity or is mediated by the enzyme or another transport protein (Russell 2003).
4-cholesten-7alpha,12alpha,24(S)-triol-3-one and NADPH + H+ react to form 5beta-cholesten-7alpha,12alpha,24(S)-triol-3-one and NADP+. This reaction is catalyzed by AKR1D1 (3-oxo-5-beta-steroid 4-dehydrogenase). AKR1D1 is localized to the cytosol, and in the course of the reaction its steroid substrate moves from the endoplasmic reticulum membrane to the cytosol. It is unclear whether this translocation results simply from its increased hydrophilicity or is mediated by the enzyme or another transport protein (Russell 2003).
5Beta-cholesten-7alpha,24(S)-diol-3-one and NADPH + H+ form 5beta-cholestan-3alpha,7alpha,24(S)-triol and NAPDP+. The reaction is catalyzed by 3alpha-hydroxysteroid dehydrogenase (AKR1C4), a cytosolic enzyme belonging to the aldo-keto reductase family (Dufort et al. 2001). Biochemical studies with rat proteins raise the possibility that other related enzymes may also carry out this reaction in vivo (Russell 2003).
The isomerization of 25(R) TetraHCA-CoA to (24R, 25R) 3alpha,7alpha,12alpha,24-tetrahydroxy-5beta-cholestanoyl-CoA, catalyzed by 2-methylacyl-CoA racemase, occurs in the peroxisomal matrix.
TetraHCA (25(R) 3alpha,7alpha,12alpha,24(S)-tetrahydroxy-5beta-cholestanoate), coenzyme A, and ATP react to form the CoA conjugate of 25(R) TetraHCA, AMP, pyrophosphate and water. This cytosolic reaction is catalyzed by SLC27A5 (BACS). SLC27A2 (VLCS) also catalyzes this reaction; the relative contributions of the two enzymes to de novo bile acid synthesis in vivo are not certain (Mihalik et al. 2002).
24-hydroxycholesterol is transported from the extracellular space to the endoplasmic reticulum. In humans, this event is the means by which the molecule, generated from cholesterol in the brain, is taken up by liver cells for conversion to bile acids and bile salts. While transport proteins are likely to play a role in this process, these proteins have not been identified (Lutjohann et al. 1996; Bjorkhem et al. 1998).
5beta-cholestan-3alpha,7alpha,12alpha,24(S)-tetrol is translocated from the cytosol to the mitochondrial matrix. The transporter that mediates its passage across the inner mitochondrial membrane is unknown: the StAR protein that performs this function for cholesterol at the start of steroid hormone biosynthesis is excluded as StAR is not expressed in liver. Other members of the START family of transporters are candidates, however (Russell 2003).
24-hydroxycholesterol is transported from the endoplasmic reticulum to the extracellular space. In humans, this event is the major source of 24-hydroxycholesterol in the blood and is the means by which the molecule, generated from cholesterol in the brain, is transported to the liver for conversion to bile acids and bile salts. While transport proteins are likely to play a role in this process, and the 24-hydroxycholesterol is likely to occur as part of a lipoprotein complex in the blood, the relevant proteins have not been identified (Lutjohann et al. 1996; Bjorkhem et al. 1998).
5beta-cholestan-3alpha,7alpha,12alpha,24(S),27-pentol, NADPH + H+, and O2 react to form 3alpha,7alpha,12alpha,24(S)-tetrahydroxy-5beta-cholestan-27-al, NADP+, and H2O. This oxidation reaction occurs in the mitochondrial matrix, catalyzed by CYP27A1.
5Beta-cholesten-7alpha,12alpha,24(S)-triol-3-one and NADPH + H+ form 5beta-cholestan-3alpha,7alpha,12alpha,24(S)-tetrol and NAPDP+. The reaction is catalyzed by 3alpha-hydroxysteroid dehydrogenase (AKR1C4), a cytosolic enzyme belonging to the aldo-keto reductase family (Dufort et al. 2001). Biochemical studies with rat proteins raise the possibility that other related enzymes may also carry out this reaction in vivo (Russell 2003).
3,7,24THCA (3alpha,7alpha,24(S)-trihydroxy-5beta-cholestanoate) is translocated from the mitochondrial matrix to the cytosol. The transporter that mediates its passage across the inner mitochondrial membrane has not been identified (Russell 2003). VLCS (SLC27A2), one of the enzymes that catalyzes CoA conjugation of 3,7,24THCA, may also be present in peroxisomes, and Mihalik et al. (2002) have hypothesized that 3,7,24THCA could be translocated unchanged from the mitochondrial matrix to the peroxisomal matrix and undergo conjugation there.
5beta-cholestan-3alpha,7alpha,12alpha,24(S)-tetrol, NADPH + H+, and O2 react to form 5beta-cholestan-3alpha,7alpha,12alpha,24(S),27-pentol, NADP+, and H2O. This reaction occurs in the mitochondrial matrix, catalyzed by CYP27A1.
Cholest-5-ene-3beta,7alpha,24(S)-triol and NAD+ react to form 4-cholesten-7alpha,24(S)-diol-3-one and NADH + H+, catalyzed by HSD3B7 (3 beta-hydroxysteroid dehydrogenase type 7) in the endoplasmic reticulum membrane. Its function in vivo has been confirmed in studies of patients with defects in bile acid synthesis (Schwarz et al. 2000).
5β-cholestan-3α,7α,24(S)-triol, NADPH + H+, and O2 react to form 5β-cholestan-3α,7α,24(S),27-tetrol, NADP+, and H2O. This reaction occurs in the mitochondrial matrix, catalyzed by CYP27A1.
5Beta-cholesten-7alpha,12a,27-triol-3-one and NADPH + H+ form 5beta-cholestan-3alpha,7alpha,12a,27-tetrol and NAPDP+. The reaction is catalyzed by 3alpha-hydroxysteroid dehydrogenase (AKR1C4), a cytosolic enzyme belonging to the aldo-keto reductase family (Dufort et al. 2001). Biochemical studies with rat proteins raise the possibility that other related enzymes may also carry out this reaction in vivo (Russell 2003).
27-hydroxycholesterol is transported from the extracellular space to the endoplasmic reticulum. In humans, this event is the means by which the molecule, generated from cholesterol in the brain, is taken up by liver cells for conversion to bile acids and bile salts (Babiker et al. 1999; Bjorkhem et al. 1994). While transport proteins are likely to play a role in this process, these proteins have not been identified.
5beta-cholestan-3alpha,7alpha,27-triol is transported from the cytosol to the mitochondrial matrix. The transporter that mediates its passage across the inner mitochondrial membrane is unknown: the StAR protein that performs this function for cholesterol at the start of steroid hormone biosynthesis is excluded as StAR is not expressed in liver. Other members of the START family of transporters are candidates, however (Russell 2003).
27-hydroxycholesterol is transported from the mitochondrial matrix to the extracellular space. In humans, this event is the major source of 27-hydroxycholesterol in the blood and is the means by which the molecule, generated from cholesterol in a variety of cell types, notably macrophages, is transported to the liver for conversion to bile acids and bile salts (Babiker et al. 1999; Bjorkhem et al. 1994). While transport proteins are likely to play a role in this process, the relevant proteins have not been identified.
Cholest-5-ene-3beta,7alpha,27-triol and NAD+ react to form 4-cholesten-7alpha,27-diol-3-one and NADH + H+, in a reaction catalyzed by HSD3B7 (3 beta-hydroxysteroid dehydrogenase type 7) in the endoplasmic reticulum membrane. Its function in vivo has been confirmed in studies of patients with defects in bile acid synthesis (Schwarz et al. 2000).
4-Cholesten-7alpha,12alpha,27-triol-3-one and NADPH + H+ react to form 5beta-cholesten-7alpha,12alpha,27-triol-3-one + NADP+. This reaction is catalyzed by AKR1D1 (3-oxo-5-beta-steroid 4-dehydrogenase). AKR1D1 is localized to the cytosol, and in the course of the reaction its steroid substrate moves from the endoplasmic reticulum membrane to the cytosol. It is unclear whether this translocation results simply from its increased hydrophilicity or is mediated by the enzyme or another transport protein (Russell 2003).
4-Cholesten-7alpha,27-diol-3-one, NADPH, and H+ react to form 5beta-cholestan-7alpha,27-diol-3-one and NADP+. This reaction is catalyzed by AKR1D1 (3-oxo-5-beta-steroid 4-dehydrogenase). AKR1D1 is localized to the cytosol, and in the course of the reaction its steroid substrate moves from the endoplasmic reticulum membrane to the cytosol. It is unclear whether this translocation results simply from its increased hydrophilicity or is mediated by the enzyme or another transport protein (Russell 2003).
5beta-cholestan-3alpha,7alpha,12alpha,27-tetrol is translocated from the cytosol to the mitochondrial matrix. The transporter that mediates its passage across the inner mitochondrial membrane is unknown: the StAR protein that performs this function for cholesterol at the start of steroid hormone biosynthesis is excluded as StAR is not expressed in liver. Other members of the START family of transporters are candidates, however (Russell 2003).
5Beta-cholesten-7alpha,27-diol-3-one and NADPH + H+ form 5beta-cholestan-3alpha,7alpha,27-triol and NAPDP+. The reaction is catalyzed by 3alpha-hydroxysteroid dehydrogenase (AKR1C4), a cytosolic enzyme belonging to the aldo-keto reductase family (Dufort et al. 2001). Biochemical studies with rat proteins raise the possibility that other related enzymes may also carry out this reaction in vivo (Russell 2003).
Sterol 12alpha hydroxylase (CYP8B1), associated with the endoplasmic reticulum membrane, catalyses the 12-alpha-hydroxylation of 4-Cholesten-7alpha,27-diol-3-one (4CHOL7a,27DONE) to 4-Cholesten-7alpha,12alpha,27-triol-3-one. While the human gene has been cloned (Gafvels et al. 1999), its protein product has not been characterised, and the enzymatic properties of human CYP8B1 protein are inferred from those of its well-characterised rabbit homolog (Ishida et al. 1992).
A molecule of extracellular glycocholate or taurocholate is transported into the cytosol, mediated by OATP-8 (SLCO1B3) in the plasma membrane. Glycocholate and taurocholate exist in the blood as complexes with serum albumin, and their uptake by OATP-8 must involve disruption of these complexes, but the molecular mechanism coupling disruption and uptake is unknown. In the body, OATP-8 is expressed on the basolateral surfaces of hepatocytes and may play a role in the uptake of glycocholate and taurocholate by the liver under physiological conditions (Kullak-Ublick et al. 2004; Trauner and Boyer 2002).
A molecule of extracellular glycocholate (GCCA) or taurocholate (TCCA) is transported into the cytosol, mediated by OATP-C (SLCO1B1) in the plasma membrane. GCCA and TCCA exist in the blood as complexes with serum albumin (ALB), and its uptake by OATP-C must involve disruption of this complex, but the molecular mechanism coupling disruption and uptake is unknown. In the body, OATP-C is expressed on the basolateral surfaces of hepatocytes and may play a role in the uptake of GCCA and TCCA by the liver under physiological conditions (Kullak-Ublick et al. 2004, Trauner & Boyer 2002).
A molecule of extracellular bile salt (glyco- or taurocholate, or glyco- or taurochenodeoxycholate) and two sodium ions are transported into the cytosol, mediated by NTCP (Na+ / taurocholate cotransporter) in the plasma membrane. Bile salts exist in the blood as complexes with serum albumin, and their uptake by NTCP must involve disruption of this complex, but the molecular mechanism of the coupling of the release of a bile salt from albumin to its uptake by NTCP is unknown. In the body, NTCP is expressed on the basolateral surfaces of hepatocytes, and this reaction is the major route by which bile salts reaborbed from the intestinal lumen into the portal circulation are recovered by the liver (Kullak-Ublick et al. 2004; Trauner and Boyer 2002).
A molecule of extracellular bile salt (glyco- or taurocholate or taurochenodeoxycholate) or bile acid (cholate or chenodeoxycholate) is transported into the cytosol, mediated by OATP-A (SLCO1A2) in the plasma membrane. Bile salts and acids exist in the blood as complexes with serum albumin, and their uptake by OATP-A must involve disruption of this complex, but the molecular mechanism coupling release of a bile salt or acid from albumin to its uptake by OATP-A is unknown. In the body, OATP-A is expressed only at low levels on the basolateral surfaces of hepatocytes and may play only a minor role in the uptake of bile salts and acids by the liver (Kullak-Ublick et al. 2004; Trauner and Boyer 2002).
A molecule of glycocholate, taurocholate, or taurochenodeoxycholate is transported from the cytosol to the extracellular space, coupled to the hydrolysis of a molecule of cytosolic ATP to ADP and orthophosphate. This reaction is mediated by ABCB3 (MRP3). In the body, this reaction mediates the release of bile salts from enterocytes into the blood (Kullak-Ublick et al. 2004; Trauner and Boyer 2002). In the cytosol, bile salts and acids are bound to a carrier protein, FABP6 (I-BABP) (Fujita et al. 1995), and in the blood these molecules are complexed with albumin. The mechanisms by which transport across the plasma membrane is coupled to release FABP6 and binding to albumin are unknown, so the entire process is annotated as a single concerted event.
A molecule of extracellular bile salt or bile acid (cholate, chenodeoxycholate, or their glycine or taurine conjugates) and a sodium ion are transported into the cytosol, mediated by ASBT (apical sodium-dependent bile acid transporter; SLC10A2) in the plasma membrane. In the body, ASBT is expressed on the apical surfaces of enterocytes, and this reaction is the first step in the process by which bile salts and acids are reaborbed from the intestinal lumen and returned to the liver (Kullak-Ublick et al. 2004, Trauner & Boyer 2002). Cytosolic ileal bile acid binding protein (IBABP, FABP6) mediates the transcellular movement of bile acids to the basolateral membrane across which they exit the cells via organic solute transporters (OST) (Kurz et al. 2003).
The nuclear orphan protein bile acid receptor aka farnesoid X-activated receptor (NR1H4 aka FXR) can be activated by bile acids and their salts, its physiological ligands. Mouse Nrih4 is highly expressed in the liver, intestine, kidney and adrenal gland, and to a lesser extent in white adipose tissue and heart (Zhang et al. 2003). Bile acids tested to activate NR1H4 are chenodeoxycholic acid (CDCA), lithocholic acid (LCHA) and deoxycholic acid (DCA) (Parks et al. 1999, Makishima et al. 1999).
Once bound to its ligand, activated NR1H4 binds to retinoic acid receptor RXR-alpha (RXRA) and either nuclear receptor coactivator 1 or 2 (NCOA1 or 2) to function as a ligand-activated transcription factor. This complex repressed transcription of the CYP7A1 gene (encoding cholesterol 7alpha-hydroxylase, the rate-limiting enzyme in bile acid synthesis) (Holt et al. 2003) and activated the SLC10A2 and 6 genes (encoding Ileal sodium/bile acid cotransporters, both bile acid transporters) (Plass et al. 2002, Ananthanarayanan et al. 2001). Thus, NR1H4 is one of the most important regulators of bile acid metabolism, regulating bile acid synthesis, conjugation, secretion and uptake (Lee et al. 2006, Houten et al. 2006).
The nuclear orphan protein bile acid receptor aka farnesoid X-activated receptor (NR1H4 aka FXR) can be activated by bile acids and their salts, its physiological ligands. Bile acids tested to activate NR1H4 are chenodeoxycholic acid (CDCA), lithocholic acid (LCHA) and deoxycholic acid (DCA) (Parks et al. 1999, Makishima et al. 1999). Once bound to its ligand, activated NR1H4 binds to retinoic acid receptor RXR-alpha (RXRA) and either nuclear receptor coactivator 1 or 2 (NCOA1 or 2) to function as a ligand-activated transcription factor. This complex repressed transcription of the CYP7A1 gene (encoding cholesterol 7alpha-hydroxylase, the rate-limiting enzyme in bile acid synthesis) (Holt et al. 2003) and activated the SLC10A2 and 6 genes (encoding Ileal sodium/bile acid cotransporters; both bile acid transporters) (Plass et al. 2002, Ananthanarayanan et al. 2001). Thus, NR1H4 is one of the most important regulators of bile acid metabolism, regulating bile acid synthesis, conjugation, secretion and uptake (Lee et al. 2006, Houten et al. 2006).
Oxysterol-binding proteins (OSBPs) and OSBP-related proteins (ORPs) comprise a 12-member mammalian gene family of intracellular lipid receptors which bind oxygenated cholesterol derivatives and are thought to mediate sterol and phospholipid synthesis (Olkkonen & Li 2013, Weber-Boyvat et al. 2013). A conserved OSBP homology domain (OHD) binds sterols and lipids and a pleckstrin homology (PH) domain and two phenylalanines in an acidic tract (FFAT) motif mediate interaction with organelle membranes (Olkkonen 2015). The primary function of the protein family remains unresolved. Most (OSBPL2,3,6,7,9 and 1A), if not all of the OSBPLs are thought to bind and coordinate distribution of oxygenated cholesterol derivatives such as 25-hydroxycholesterol (25OH-CHOL). Oxysterol-binding protein 2 (OSBP2, aka ORP4) is primarily expressed in brain, and to a lesser extent in heart, skeletal muscle, spleen and kidney. It has a diffuse cytoplasmic localisation profile and is a high-affinity 25OH-CHOL binding protein (Wang et al. 2002, Suchanek et al. 2007). RNAi silencing of all OSBP2 variants in HEK293 and HeLa cells resulted in growth arrest but not cell death, suggesting OSBP2 is essential for cell proliferation and survival (Charman et al. 2014).
Oxysterol-binding protein 1 (OSBP) is involved in lipid countertransport between the Golgi and the ER membranes. It specifically exchanges sterols such as 25-hydroxycholesterol (25OH-CHOL) with phosphatidylinositol 4-phosphate (PI4P), delivering it to the Golgi in exchange for PI4P, which is degraded by the SAC1/SACM1L phosphatases in the ER. The PH-FFAT region of OSBP is necessary for membrane tethering, which then directs forward sterol and backward PI4P transport by the ORD domain of OSBP (Mesmin et al. 2013).
StAR-related lipid transfer protein 5 (STARD5) is highly expressed in at least three cell types targeted by bile acids: liver macrophage Kupffer cells, peripheral macrophages and kidney proximal tubule cells. Unlike the other members of the STARD4 subfamily of the START-containing proteins (members 4 and 6), STARD5 possesses low or no affinity for cholesterol but instead, specifically binds primary and secondary bile acids. STARD5's affinity for the secondary bile acids deoxycholic acid (DCA) and lithocholic acid (LCA), which lack the alpha-OH in C7, is greater than that of the primary bile acids cholic acid (CA) and chenodeoxycholic acid (CDCA) (Letourneau et al. 2012, Letourneau et al. 2013). The physiological roles played by STARD5 remain to be elucidated but it could act as a bile acid transporter or sensor.
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25R)
3alpha,7alpha,12alpha,24-tetrahydroxy-5beta-cholestanoyl-CoAand acid (OATP-A)
complexcomplexed with
albuminAnnotated Interactions
25R)
3alpha,7alpha,12alpha,24-tetrahydroxy-5beta-cholestanoyl-CoA25R)
3alpha,7alpha,12alpha,24-tetrahydroxy-5beta-cholestanoyl-CoA25R)
3alpha,7alpha,12alpha,24-tetrahydroxy-5beta-cholestanoyl-CoAOnce bound to its ligand, activated NR1H4 binds to retinoic acid receptor RXR-alpha (RXRA) and either nuclear receptor coactivator 1 or 2 (NCOA1 or 2) to function as a ligand-activated transcription factor. This complex repressed transcription of the CYP7A1 gene (encoding cholesterol 7alpha-hydroxylase, the rate-limiting enzyme in bile acid synthesis) (Holt et al. 2003) and activated the SLC10A2 and 6 genes (encoding Ileal sodium/bile acid cotransporters, both bile acid transporters) (Plass et al. 2002, Ananthanarayanan et al. 2001). Thus, NR1H4 is one of the most important regulators of bile acid metabolism, regulating bile acid synthesis, conjugation, secretion and uptake (Lee et al. 2006, Houten et al. 2006).
and acid (OATP-A)
complexcomplexed with
albumin