Formation of chenodeoxycholic acid from 3 alpha, 7 alpha-dihydroxy-5 beta-cholestanoic acid by rat liver peroxisomes

The oxidation of the side chain of 3 alpha, 7 alpha-dihydroxy-5 beta-cholestanoic acid (DHCA) into chenodeoxycholic acid has been studied in subcellular fractions of rat liver. The product was separated from the substrate by high pressure liquid chromatography and identified by gas-liquid chromatography-mass spectrometry. The highest specific rate of conversion was found in the heavy (M) and the light (L) mitochondrial fractions with the highest enrichment in the L fraction. Washing the M fraction reduced the side chain cleavage activity by 90%. The peroxisomal marker enzyme urate oxidase was reduced to the same extent. The activity found in the M fraction may thus be due to peroxisomal contamination. After centrifugation of the L fraction on a Nycodenz density gradient, the highest specific activity for side chain cleavage of DHCA (31 nmol X mg-1 X h-1) was found in the fraction with the highest peroxisomal marker enzyme activity. This fraction also catalyzed conversion of 3 alpha,7 alpha,12 alpha-5 beta-cholestanoic acid (THCA) into cholic acid at the highest rate (32 nmol X mg-1 X h-1). The peroxisomal oxidation of DHCA into chenodeoxycholic acid required the presence of ATP, CoA, Mg2+, and NAD in the incubation medium. The reaction was not inhibited by KCN. It is concluded that rat liver peroxisomes contain enzymes able to catalyze the cleavage of the side chain of both DHCA and THCA. The enzymes involved are similar to, but not necessarily identical to, those involved in the peroxisomal beta-oxidation of fatty acids.