24-Hydroxycholesterol is a substrate for hepatic cholesterol 7 alpha-hydroxylase (CYP7A)

(24S)-Hydroxycholesterol is formed from cholesterol in the brain and is imp ortant for cholesterol homeostasis in this organ, Elimination of (24S)-hydr oxycholesterol has been suggested to occur in the liver but little is known about the metabolism of this oxysterol, In the present investigation, we r eport formation of 7 alpha,24-dihydroxycholesterol in pig and human liver. 7 alpha-hydroxylase activity toward both isomers of 24-hydroxycholesterol [ (24S) and (24R)] was found in a partially purified and reconstituted choles terol 7 alpha-hydroxylase (CYP7A) enzyme fraction from pig liver microsomes , In contrast, a purified enzyme fraction of pig liver oxysterol 7 alpha-hy droxylase with high activity toward 27-hydroxycholesterol did not show any detectable activity toward 24-hydroxycholesterol. 7 alpha-Hydroxylation of 24-hydroxycholesterol was strongly inhibited by 7-oxocholesterol, a known i nhibitor of CYP7A, Human CYP7A, recombinantly expressed in Escherichia coli and in simian COS cells, showed 7 alpha-hydroxylase activity toward both c holesterol and the two isomers of 24-hydroxycholesterol, with a preference for the (24S)-isomer. Our results show that 24-hydroxycholesterol is metabo lized by CYP7A an enzyme previously considered to be specific for cholester ol and cholestanol and not active toward oxysterols, Because CYP7A is the r ate-limiting enzyme in the major pathway of bile acid biosynthesis, the pos sibility is discussed that at least part of the 24-hydroxycholesterol is co nverted into 7 alpha-hydroxylated bile acids by the enzymes involved in the normal biosynthesis of bile acids.