The molecular basis of isolated 17,20 lyase deficiency

Human P450c17 catalyzes the 17 alpha-hydroxylation of pregnenolone to 17OH pregnenolone and of progesterone to 17 alpha-OH progesterone; the same P450 c17 enzyme also catalyzes 17,20 lyase activity on the same active site, con verting 17OH-pregnenolone to DHEA. Rodent and porcine P450c17 also catalyze 17,20 lyase activity with Delta(4) substrates, converting 17OH-progesteron e to Delta(4) androstenedione, but human P450e 17 catalyzes this reaction v ery inefficiently, so that virtually all human C19 sex steroids are made vi a 17OH pregnenolone and DHEA. P450c17 is encoded by a single gene and a sin gle species of mRNA. Many mutations of this gene have been described, but u ntil recently all of these either entirely eliminated both 17 alpha-hydroxy lase and 17,20 lyase activity, or affected each activity equivalently. We h ave identified and characterized the first patients with P450c17 mutations that selectively ablate 17,20 lyase activity while retaining 17 alpha-hydro xylase activity. Through a combination of enyzmologic experiments in transf ected mammalian cells and in genetically manipulated yeast, plus a computer model of human P450e 17, we have proven that the responsible mutations, R3 47H and R358Q lie in the redox-partner binding site of P450c17. This site, through which P450e 17 interacts with P450 oxidoreductase to receive the el ectrons needed for catalysis, can be allostericly influenced by cytochrome b(5). These two mutations have contributed substantially to our understandi ng of the mechanisms by which 17 alpha-hydroxylase and 17,20 lyase activiti es are regulated independently, and thus have contributed to the study of r egulated 17,20 lyase activity in adrenarche, aging, and the polycystic ovar y syndrome.