Presence of individual enzymes of cholesterol biosynthesis in rat liver peroxisomes

Abstract

Cholesterol biosynthesis by isolated rat liver peroxisomes was examined. Labeling of cholesterol from [ 3H]mevalonate in the presence of peroxisomes required the addition of cytosol, since peroxisomes, like microsomes, apparently possess only those enzymes of cholesterol biosynthesis subsequent to the steps involving farnesyl-PP. Under the conditions employed the amounts of 4,4-dimethyl and desmethyl sterols generated by peroxisomes were equal to or exceeded those produced by the microsomes. In addition, marker enzyme analysis demonstrated minimal microsomal contamination in the peroxisomal fraction. The metabolite patterns observed by HPLC after incubation of these two fractions with [ 3H]mevalonate were different. Dihydrolanosterol oxidase, steroid-14-reductase, steroid-8-isomerase, and steroid-3-ketoreductase activities were present in peroxisomes. Separation of peroxisomes into membranes and contents revealed that all the synthesizing activities are associated with the membrane fraction. 7α-Hydroxylase, which catalyzes the first step in the biosynthesis of bile acids, was also present in peroxisomes, but it remains to be clarified to what extent peroxisomal cholesterol is a substrate for bile acid synthesis.