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The rate of oxidation of cholesterol and its analogues to pregnenolone (3beta-hydroxypregn-5-en-20-one) by various mitochondrial preparations was measured. Sterols with the cholest-5-en-3beta-ol ring system and saturated side chains of different lengths were converted into pregnenolone rat rates similar to that of cholesterol. This marked lack of mitochondrial specificity towards the steroid side chains is in direct contrast with the rat liver microsomal cholesterol 7alpha-hydroxylase, which has a high specificity for the side chain. Steroids that retain the ring system, but contain hydroxyl groups at various points in the side chain, are converted into pregnenolone at rates three to eight times higher than in cholesterol. The results are discussed with reference to current ideas on the mechanism of the side-chain cleavage of cholesterol. The results are discussed with reference to current ideas on the mechanism of the side-chain cleavage of cholesterol.

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