Cholesterol biosynthesis from lanosterohdevelopment of a novel assay method and characterization of rat liver microsomal tanosterol Δ24-reductase

The membrane-bound sterol A24-reductase(24-reductase)catalyzes anaerobic reduct ion of the 24(25)-cues of lanosterol and other obligatory intermediates of cholesterol biosynthesis from lanosterol, iX novel assay method and properties of the 24-reductase are described. More than a 120-fold induction of the 24reductase a(tivity was achieved by feeding rats a diet containing 5%eholestyramine plus 0. l%lovastatin in chow and by modulating diurnal variation. With this enzyme induction condition, lanosterol was converted efficiently to dihydrolanosterol in both intact hepatic microsomes and freshly isolated hepatocytes only when either miconazole or carbon monoxide was added to inhibit i4o-demethylation of lanosterol. AR45 cells, which are deficient in 14a-methyl demethylase(1.10-l)M),exhibit lanosterol 24-reductase activity without addition of either carbon monoxide or miconazole. Conversely,inhibition of the 24-reductase was no1 required for the expression of 14a-DM activity. Studies on the substrata specificities for the 24-reductase using different 24(25)-enes showed that the me)st reactive substrate was 5a-cholesta-7,24-dien- 33-ol which has exhibiled a maximal 18-fold higher K, than that of lanosterol without aid of 14a-l)M inhibitor. Based on our new findings, we suggest that reduction of sterols should take place primarily right after sterol As somerization of zymosterol in the 19-step pathway of cholesterol biosynthesis from lansslorol.