24-hydroxycholesterol is a substrate for hepatic cholesterol 7alpha-hydroxylase (CYP7A).
(24S)-Hydroxycholesterol is formed from cholesterol in the brain and is important for cholesterol homeostasis in this organ. Elimination of (24S)-hydroxycholesterol has been suggested to occur in the liver but little is known about the metabolism of this oxysterol. In the present investigation, we report formation of 7alpha, 24-dihydroxycholesterol in pig and human liver. 7alpha-hydroxylase activity toward both isomers of 24-hydroxycholesterol [(24S) and (24R)] was found in a partially purified and reconstituted cholesterol 7alpha-hydroxylase (CYP7A) enzyme fraction from pig liver microsomes. In contrast, a purified enzyme fraction of pig liver oxysterol 7alpha-hydroxylase with high activity toward 27-hydroxycholesterol did not show any detectable activity toward 24-hydroxycholesterol. 7alpha-Hydroxylation of 24-hydroxycholesterol was strongly inhibited by 7-oxocholesterol, a known inhibitor of CYP7A. Human CYP7A, recombinantly expressed in Escherichia coli and in simian COS cells, showed 7alpha-hydroxylase activity toward both cholesterol and the two isomers of 24-hydroxycholesterol, with a preference for the (24S)-isomer. Our results show that 24-hydroxycholesterol is metabolized by CYP7A, an enzyme previously considered to be specific for cholesterol and cholestanol and not active toward oxysterols. Because CYP7A is the rate-limiting enzyme in the major pathway of bile acid biosynthesis, the possibility is discussed that at least part of the 24-hydroxycholesterol is converted into 7alpha-hydroxylated bile acids by the enzymes involved in the normal biosynthesis of bile acids.
Norlin
M
M
Division of Biochemistry, Department of Pharmaceutical Biosciences, University of Uppsala, S-751 23 Uppsala, Sweden.
Toll
A
A
Björkhem
I
I
Wikvall
K
K
D000818:Animals; D002784:Cholesterol / Q000378:metabolism; D002790:Cholesterol 7-alpha-Hydroxylase / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000378:metabolism*; D002849:Chromatography, Gas; D003577:Cytochrome P-450 Enzyme System / Q000302:isolation & purification / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D006888:Hydroxycholesterols / Q000378:metabolism*; D006900:Hydroxylation; D007653:Ketocholesterols / Q000494:pharmacology; D008099:Liver / Q000201:enzymology / Q000648:ultrastructure; D008861:Microsomes / Q000201:enzymology; D011994:Recombinant Proteins / Q000302:isolation & purification / Q000378:metabolism; D013379:Substrate Specificity; D013552:Swine; D014162:Transfection
D004791:Enzyme Inhibitors; D006888:Hydroxycholesterols; D007653:Ketocholesterols; D011994:Recombinant Proteins; C044563:24-hydroxycholesterol; C076996:27-hydroxycholesterol; D003577:Cytochrome P-450 Enzyme System; D002784:Cholesterol; D002790:Cholesterol 7-alpha-Hydroxylase; C003001:7-ketocholesterol