Structure and functions of human oxysterol 7 alpha-hydroxylase cDNAs and gene CYP7B1

Oxysterol 7 alpha-hydroxylase has broad substrate specificity for sterol me tabolites and may be involved in many metabolic processes including bile ac id synthesis and neurosteroid metabolism, The cloned human oxysterol 7 alph a-hydroxylase (CYP7B1) cDNA encodes a polypeptide of 506 amino acid residue s that shares 40% sequence identity to human cholesterol 7 alpha-hydroxylas e (CW7A1), the rate-limiting enzyme in the conversion of cholesterol to bil e acids in the liver. In contrast to the liver-specific expression of CYP7A 1, CYP7B1 mRNA transcripts were detected in human tissues involved in stero id genesis (brain, testes, ovary, and prostate) and in bile acid synthesis (liver) and reabsorption (colon, kidney, and small intestine). The human ox ysterol 7 alpha-hydroxylase transiently expressed in 293/T cells was able t o catalyze 7 alpha-hydroxylation of 27-hydroxycholesterol and dehydroepiand rosterone (DHEA), The human CYP7A1 and CYP7B1 both contain six exons and fi ve introns. However, CPP7B1 spans at least 65 kb of the genome and is about 6-fold longer than CYP7A1. The transcription start site (+1) was localized 204 bp upstream of the initiation codon, No TATA box-like sequence was fou nd near the transcription start site. Transient transfection assays of CYP7 B1 promoter/luciferase reporter constructs in HepG2 cells revealed that the promoter was highly active. The 5' upstream region from nt -83 to +189 is the core promoter of the gene.