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Evidence suggesting that more than 1 sterol side chain cleavage enzyme system exists in mitochondria from bovine adrenal cortex

, : Evidence suggesting that more than 1 sterol side chain cleavage enzyme system exists in mitochondria from bovine adrenal cortex. Journal of Biological Chemistry 254(10): 4096-4100

The kinetics of cleavage of the side chains of 3 possible sterol precursors of the steroid hormones, cholesterol, cholesterol sulfate and cholesterol acetate, by mitochondria prepared from bovine adrenal cortex were measured under various conditions in an effort to discern whether 1 or more enzyme systems catalyze their oxidation. Kinetic experiments revealed that each substrate exhibited 2 Km values. This is the result that would be expected if there were 2 enzymes, each of which has high affinity for 1 sterol substrate and low affinity for the others. Each substrate inhibited the oxidation of the others competitively, but with a much higher Ki [Km for 2nd substrate] than would be expected if they were competing for a single site. Additivity experiments indicated that there are independent sites for cholesterol and cholesterol sulfate and also for cholesterol and cholesterol acetate. Cholesterol sulfate and cholesterol acetate appeared to be oxidized at the same, or at least interfering, sites. Inhibition by other steroids indicated that there are specific inhibitors for the different enzymatic activities. For example, dehydroisoandroesterone sulfate inhibits only the cleavage of cholesterol sulfate. Incubation in the presence of the solvents, methanol, ethanol, or acetone, led to destruction of the enzymatic activity toward cholesterol sulfate and cholesterol acetate, but cholesterol-cleaving activity was not affected by methanol and was only partially inhibited by the other solvents. Cholesterol is not the only sterol precursor of the steroid hormones and each class of precursor has a specific array of enzymes that convert it into its final steroidal product.

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