The Enzyme Database

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Accepted name: 5β-cholestane-3α,7α-diol 12α-hydroxylase
Reaction: 5β-cholestane-3α,7α-diol + [reduced NADPH—hemoprotein reductase] + O2 = 5β-cholestane-3α,7α,12α-triol + [oxidized NADPH—hemoprotein reductase] + H2O
For diagram of cholesterol catabolism (rings A, B and C), click here
Other name(s): 5β-cholestane-3α,7α-diol 12α-monooxygenase; sterol 12α-hydroxylase (ambiguous); CYP8B1; cytochrome P450 8B1
Systematic name: 5β-cholestane-3α,7α-diol,[reduced NADPH—hemoprotein reductase]:oxygen oxidoreductase (12α-hydroxylating)
Comments: A cytochrome P-450 (heme-thiolate) protein found in mammals. This is the key enzyme in the biosynthesis of the bile acid cholic acid (3α,7α,12α-trihydroxy-5β-cholanoic acid). The activity of this enzyme determines the biosynthetic ratio between cholic acid and chenodeoxycholic acid [3]. The enzyme can also hydroxylate the substrate at the 25 and 26 position, but to a lesser extent [1].
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc
1.  Hansson, R. and Wikvall, K. Hydroxylations in biosynthesis of bile acids. Cytochrome P-450 LM4 and 12α-hydroxylation of 5β-cholestane-3α,7α-diol. Eur. J. Biochem. 125 (1982) 423–429. [DOI] [PMID: 6811268]
2.  Hansson, R. and Wikvall, K. Hydroxylations in biosynthesis and metabolism of bile acids. Catalytic properties of different forms of cytochrome P-450. J. Biol. Chem. 255 (1980) 1643–1649. [PMID: 6766451]
3.  Lundell, K. and Wikvall, K. Gene structure of pig sterol 12α-hydroxylase (CYP8B1) and expression in fetal liver: comparison with expression of taurochenodeoxycholic acid 6α-hydroxylase (CYP4A21). Biochim. Biophys. Acta 1634 (2003) 86–96. [DOI] [PMID: 14643796]
4.  del Castillo-Olivares, A. and Gil, G. α1-Fetoprotein transcription factor is required for the expression of sterol 12α -hydroxylase, the specific enzyme for cholic acid synthesis. Potential role in the bile acid-mediated regulation of gene transcription. J. Biol. Chem. 275 (2000) 17793–17799. [DOI] [PMID: 10747975]
5.  Yang, Y., Zhang, M., Eggertsen, G. and Chiang, J.Y. On the mechanism of bile acid inhibition of rat sterol 12α-hydroxylase gene (CYP8B1) transcription: roles of α-fetoprotein transcription factor and hepatocyte nuclear factor 4alpha. Biochim. Biophys. Acta 1583 (2002) 63–73. [DOI] [PMID: 12069850]
6.  Russell, D.W. The enzymes, regulation, and genetics of bile acid synthesis. Annu. Rev. Biochem. 72 (2003) 137–174. [DOI] [PMID: 12543708]
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