Cholesterol 7α-hydroxylase in rat liver microsomal preparations

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Abstract
Subcellular fractions containing microsomes prepared from rat livers homogenized in the absence of EDTA catalysed the oxidation of cholesterol to 7α-hydroxycholesterol, 7-oxocholesterol, 7β-hydroxycholesterol and 5α-cholestane-3β,5,6β-triol. These reactions required native protein, molecular oxygen and NADPH. It is suggested that these compounds are formed by a peroxidation analogous to the peroxidation of fatty acids catalysed by liver microsomal preparations. Incubations of [4-14C]cholesterol with microsomal preparations from rat liver homogenized in the presence of EDTA gave 7α-hydroxy[14C]cholesterol as the main product. This reaction required molecular oxygen and NADPH, and was inhibited by CO. The mass of 7α-hydroxycholesterol formed during the incubation was measured by a double-isotope-derivative dilution procedure. This procedure was used to assay the activity of cholesterol 7α-hydroxylase and to measure low concentrations of endogenous 7α-hydroxycholesterol in liver.