Cytochrome P-450 C 21scc: One enzyme with two actions: Hydroxylase and lyase

Abstract

Testis, adrenal, ovary and placenta contain a microsomal cytochrome P-450 that is capable of converting progesterone to androstenedione and pregnenolone to dehydroepi-androsterone. This conversion requires 17α-hydroxylation followed by C 17,20-lyase activity which are both catalyzed by this one protein. Gene cloning and Northern blotting reveal that, at least in man, the same gene is responsible for both testicular and adrenal enzymes. The enzyme was first purified from neonatal pig testis. Both the testicular and adrenal enzymes show a marked preference for the 5-ene substrate (pregnenolone) in keeping with the extensive use of the 5-ene pathway in that species. Affinity alkylation with 17α-bromoacetoxyprogesterone reveals a conserved cysteine at the active site of the enzyme and confirms the conclusion that a single enzyme catalyzes both reactions. Under some circumstances the enzyme catalyzes only 17α-hydroxylation to permit the formation of the C 21 steroid cortisol. The regulation of lyase activity, i.e. the determination of the extent to which the second activity is expressed, results from the availability of P-450 reductase. No doubt the greater concentration of this protein in testicular as opposed to adrenal microsomes (× 3.5) is responsible for the production of androgens in the testis and cortisol in the adrenal. Testicular cytochrome b 5 also specifically stimulates lyase activity and also causes the porcine enzyme to catalyze a new reaction, i.e. Δ 16-synthetase, resulting in synthesis of the important pheromone androsta-4,16-dien-3 one from progesterone.