Effect of the side-chain structure on the specificity of beta-oxidation in bile acid biosynthesis in rat liver homogenates

Abstract

3Alpha, 7alpha, 12alpha-trihydroxy-5beta-cholestan-26-oic acid (C27-5beta-cholestanoic acid) derivatives with different carbon-number side chains were incubated with rat liver 800 g supernatant to study the effect of the side-chain length on the beta-oxidation system in bile acid biosynthesis. The intermediate alpha, beta-unsaturated and beta-hydroxylated bile acids, and the corresponding degradation products, were quantitatively determined by gas chromatography. The longer side-chained derivatives (C28- and C29-5beta-cholestanoic acids) were converted into corresponding sidechain degradation products, and the alpha,beta-unsaturated and beta-hydroxylated intermediates were also produced. On the other hand, the shorter side-chained derivative (C26-5beta-cholestanoic acid) only gave alpha,beta-unsaturated intermediate. The total formation of intermediates and degradation products from corresponding substrates was in the order of C27- > C28- > C29- > C26-5beta-cholestanoic acids. In the case of clofibrate-treated rat liver 800 g supernatant, the formation of intermediates and final degradation products from C28- and C29-5beta-cholestanoic acids increased significantly. These longer side-chained analogues seemed to be subjected to beta-oxidation system(s) induced with clofibrate treatment. The effect of a terminal methyl group in the side chain of 5beta-cholestanoic acid on the oxidation system was also investigated using 3alpha, 7alpha, 12alpha-trihydroxy-27-nor-5beta-cholestanoic acid derivatives as enzymatic substrates. These derivatives gave corresponding side chain degradation products, but the formation of intermediates was not detected. The formation of side chain cleavage products from 27-nor-5beta-cholestanoic acid derivatives increased to 10 to 25-fold that of the controls by treatment with clofibrate. The results suggested that the beta-oxidation system for 27-nor-5beta-cholestanoic acid derivatives was different from that for C27-5beta-cholestanoic acid, despite their bile acid steroidal structure.