Partial purification and characterization of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol and 5 beta-cholestane-3 alpha, 7 alpha-diol 27-monooxygenase

Distinct binding of cholesterol and 5beta-cholestane-3alpha,7alpha,12alpha-triol to cytochrome P450 27A1: evidence from modeling and site-directed mutagenesis studies

Cytochrome P450 27A1 (P450 27A1 or CYP27A1) is an important enzyme that participates in different pathways of cholesterol degradation as well as in the activation of vitamin D(3). Several approaches were utilized to investigate how two physiological substrates, cholesterol and 5beta-cholestane-3alpha,7alpha,12alpha-triol, interact with CYP27A1. The enzyme active site was first probed spectrally by assessing binding of the two substrates and five substrate analogues followed by computer modeling and site-directed mutagenesis. The computer models suggest that the spatial positions and orientations of cholesterol and 5beta-cholestane-3alpha,7alpha,12alpha-triol are different in the enzyme active site. As a result, some of the active site residues interact with both substrates, although they are situated differently relative to each steroid, and some residues bind only one substrate. Mutation of the overlapping substrate-contact residues (W100, H103, T110, M301C, V367, I481, and V482) affected CYP27A1 binding and enzyme activity in a substrate-dependent manner and allowed identification of several important side chains. T110 is proposed to interact with the 12alpha-hydroxyl of 5beta-cholestane-3alpha,7alpha,12alpha-triol, whereas V367 seems to be crucial for correct positioning of the cholesterol C26 methyl group and for regioselective hydroxylation of this substrate. Distinct binding of the CYP27A1 substrates may provide insight into why phenotypic manifestations of cerebrotendinous xanthomatosis, a disease associated with CYP27A1 deficiency, are so diverse.

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.