Biochemical differences between rat and human cytochrome P450c17 support the different steroidogenic needs of these two species

Microsomal 17 alpha-hydroxylase/17,20-lyase cytochrome P450 (P450c17)cataly zes both the 17 alpha-hydroxylase reaction required to produce cortisol, th e major glucocorticoid in many animals, and the 17,20-lyase activity requir ed for the production of androgens in all animals. In rodents such as rat, which utilize corticosterone as the major glucocorticoid, P450c17 is expres sed predominantly in the gonads, and is absent in the adrenal. In other spe cies including humans, P450c17 is expressed in both adrenal and gonads and participates in both glucocorticoid and androgen production. Rat and human forms of P450c17 are 69% identical at the amino acid level. Based on the di fferences in physiological roles between P450c17 in these two species, it c ould be predicted that major differences would be observed in their hydroxy lase activities. Contrary to this hypothesis, using partially purified, rec ombinant human and rat P450c17, we found that the most significant differen ces lie in their lyase activities, Lyase activities demonstrate that the sa t enzyme favors Delta(4) (progesterone) substrates while the human enzyme f avors Delta(5) (pregnenolone) substrates. This substrate preference is also observed in the ability of steroids to decrease uncoupled H2O2 production and to increase stability during turnover. Cytochrome bs, a microsomal elec tron-transfer protein, enhances lyase activities of rat and human P450c17. However, the most dramatic stimulatory effect is an the human BO-PROG lyase activity. This enhancement of activities is not associated with electron t ransfer. These differences in biochemical properties between the two forms of P450c17 indicate that human P450c17 has evolved as an enzyme system that limits androgen production to the gonads where a favorable b(5):P450c17 ra tio exists. Even though orthologous forms of P450c17 are capable of catalyz ing the same enzymatic activities, specific physiological requirements of e ach species ensure biochemical differences between these enzymes.