Comparative studies on omega-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol in the mitochondrial and microsomal fraction of the liver from several vertebrates.

The activity and the stereospecificity of omega-hydroxylation, a hydroxylation at one of the two terminal methyl groups of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol, which is thought to be the first step in side-chain degradation resulting in the formation of cholic acid, was elucidated in mitochondria and microsomes of the liver from several evolutionarily primitive vertebrates, fish, frogs, turtles, and chickens in addition to such mammals as rats. hamsters, and rabbits. The detection of omega-hydroxylation products (25R)- and (25S)-5 beta-cholestane-3 alpha, 7 alpha, 12 alpha, 26-tetrols as well as the separation of their two isomers was facilitated using high-performance liquid chromatography after conversion to 9-anthroyl derivatives. All the mammals examined, except for the rat, exhibited predominant activity in the mitochondrial fraction. Although the hydroxylation activity was somewhat lower in the primitive vertebrates, it was present in the mitochondria more than in the microsomes. Furthermore, the stereospecific formation of a 25R-isomer was detected in the mitochondrial fraction of most animals estimated. However, activity in the carp liver was seven times higher in the microsomes than in the mitochondria, and the hydroxylation product was almost always a 25R-isomer. Omega-Hydroxylation activity could not be detected in rainbow trout, suggesting the existence of another biosynthetic pathway, not via 26-hydroxylation, as in the 25-hydroxylation pathway, for the production of bile acid.