Lifetime of microsomal cytochrome P-450 and steroidogenic enzymes in rat testis as influenced by human chorionic gonadotrophin |
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Authors: | J L Purvis J A Canick S A Latif J H Rosenbaum J Hologgitas R H Menard |
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Institution: | Department of Biochemistry, University of Rhode Island, Kingston, Rhode Island, 02881 U.S.A. |
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Abstract: | The lifetime of different microsomal steroidogenic enzymes and the cytochrome components of the NADPH-cytochrome P-450 pathway have been determined in rat testis by measuring their decrease logarithmically after hypophysectomy. Although both cytochrome P-450 and 17α-hydroxylase show biphasic decay curves, the first decay curve contains 89–94% of the cytochrome P-450 and 17α-hydroxylase levels. Steroidogenic enzymes which are located mainly in the leydig cells, decay much faster than microsomal protein, days, which represents mainly decay of tubular protein. The similarity between the major half-life of cytochrome P-450, days, 17α-hydroxylase, days and the C17–C20 lyase, days and the uniformity of their response to human chorionic gonadotrophin (HCG) provides additional evidence that these two steroidogenic enzymes require cytochrome P-450. Both the 17α-hydroxylase and the C17–C20 lyase were shown to have a constant activity per nmole of cytochrome P-450 during a sixfold change in the level of cytochrome P-450 brought about by HCG treatment of rats with intact pituitaries. The decay of 17β-hydroxysteroid dehydrogenase, days, was slower than P-450 dependent enzymes. Rats with intact pituitaries are not under maximal stimulation by endogenous LH because addition of HCG increases the levels of microsomal and mitochondrial cytochrome P-450 220 and 1620%, respectively. The rates of synthesis during the increase from one cytochrome P-450 level to another was calculated at testes/day for microsomal cytochrome P-450 and 0.10 nmoles/2 testes/day for mitochondrial cytochrome P-450. Treatment of hypophysectomized rats with HCG results in large increases of cytochrome P-450, 17α-hydroxylase, C17–C20 lyase and 5α-reductase, but not cytochrome b5, microsomal protein, 7α-hydroxylase, or the 17β-hydroxysteroid dehydrogenase. While it is clear that the two cytochrome P-450 dependent hydroxylases involved in steroidogenesis and the 5α-reductase are under the control of gonadotrophin, it is not clear how 17β-hydroxysteroid dehydrogenase levels are maintained or in what manner the 5α-reductase level is controlled in mature animals. |
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