7α-Hydroxylation of 26-hydroxycholesterol, 3β-hydroxy-5-cholestenoic acid and 3β-hydroxy-5-cholenoic acid by cytochrome P-450 in pig liver microsomes

AbstractPig liver microsomes were found to catalyze the 7α-hydroxylation of several potential bile acid precursors besides cholesterol. 26-Hydroxycholesterol, 3β-hydroxy-5-cholestenoic acid and 3β-hydroxy-5-cholenoic acid were all efficiently converted into the 7α-hydroxylated products. Two cytochrome P-450 fractions showing 7α-hydroxylase activity could be isolated. One fraction catalyzed 7α-hydroxylation of 26-hydroxycholesterol. 3β-hydroxy-5-cholestenoic acid and 3β-hydroxy-5-cholenoic acid but was inactive towards cholesterol. The other fraction catalyzed 7α-hydroxylation of cholesterol in addition to the other substrates. 26-Hydroxycholesterol in equimolar concentration did not inhibit the cholesterol 7α-hydroxylase activity of this fraction. It is concluded that liver microsomes contain a cytochrome P-450 catalyzing 7α-hydroxylation of 26-hydroxycholesterol, 3β-hydroxy-5-cholestenoic acid and 3β-hydroxy-5-cholenoic acid. The results indicate that this cytochrome P-450 is different from that catalyzing 7α-hydroxylation of cholesterol

CYP7A1 (cytochrome P450, family 7, subfamily A, polypeptide 1)

Review on CYP7A1 (cytochrome P450, family 7, subfamily A, polypeptide 1), with data on DNA, on the protein encoded, and where the gene is implicated

Enzymatic activation in vitamin D signaling : Past, present and future

Vitamin D signaling is important in regulating calcium homeostasis essential for bone health but also displays other functions in cells of several tissues. Disturbed vitamin D signaling is linked to a large number of diseases. The multiple cytochrome P450 (CYP) enzymes catalyzing the different hydroxylations in bioactivation of vitamin D3 are crucial for vitamin D signaling and function. This review is focused on the progress achieved in identification of the bioactivating enzymes and their genes in production of 1α,25-dihydroxyvitamin D3 and other active metabolites. Results obtained on species- and tissue-specific expression, catalytic reactions, substrate specificity, enzyme kinetics, and consequences of gene mutations are evaluated. Matters of incomplete understanding regarding the physiological roles of some vitamin D hydroxylases are critically discussed and the authors will give their view of the importance of each enzyme for vitamin D signaling. Roles of different vitamin D receptors and an alternative bioactivation pathway, leading to 20-hydroxylated vitamin D3 metabolites, are also discussed. Considerable progress has been achieved in knowledge of the vitamin D3 bioactivating enzymes. Nevertheless, several intriguing areas deserve further attention to understand the pleiotropic and diverse activities elicited by vitamin D signaling and the mechanisms of enzymatic activation necessary for vitamin D-induced responses

Hormonal regulation of the human sterol 27-hydroxylase gene CYP27A1.

The nucleotide sequence data reported in this paper will appear in EMBL Nucleotide Sequence Database under the accession number AJ 544720. The mitochondrial sterol 27-hydroxylase (CYP27A1) is a multifunctional cytochrome P450 enzyme that catalyses important hydroxylations in the biosynthesis of bile acids and bioactivation of vitamin D(3). Previous results [Babiker, Andersson, Lund, Xiu, Deeb, Reshef, Leitersdorf, Diczfalusy and Bj örkhem (1997) J. Biol. Chem. 272, 26253-26261] suggest that CYP27A1 plays an important role in cholesterol homoeostasis and affects atherogenesis. In the present study, the regulation of the human CYP27A1 gene by growth hormone (GH), insulin-like growth factor-1 (IGF-1), dexamethasone, thyroid hormones and PMA was studied. HepG2 cells were transfected transiently with luciferase reporter gene constructs containing DNA fragments flanking the 5'-region of the human CYP27A1 gene. GH, IGF-1 and dexamethasone increased the promoter activity by 2-3-fold, whereas thyroxine (T(4)) and PMA repressed the activity significantly when measured with luciferase activity expressed in the cells. The endogenous CYP27A1 enzyme activity in the cells was stimulated by GH, IGF-1 and dexamethasone, whereas T(4) and PMA inhibited the activity. Experiments with progressive deletion/luciferase reporter gene constructs indicated that the response elements for GH may be localized in a region upstream to position -1094 bp. The putative response elements for dexamethasone were mapped to positions between -792 and -1095 bp. The -451 bp fragment of the human CYP27A1 gene was found to confer the activation by IGF-1, and the inhibition by T(4) and PMA. Results of the present study suggest that CYP27A1 is regulated in human cells by hormones and signal-transduction pathways