Steroid 17,20-lyase from testis microsomes: participation of NADPH cytochrome c reductase

The enzymic activity of testis microsomes which mediates cleavage of the 2-carbon side chain from the 17-position of 21-carbon steroids is a mixed-function oxidase and recent reports have suggested that cytochrome P-450 is a participant in this reaction. The studies reported in this communication were intended to demonstrate that the flavoprotein, NADPH cytochrome c reductase, is also a participant in the reaction.The cleavage activity (referred to here as 17,20-lyase) from rat testis microsomes was shown to be inhibited by a number of agents which are electron acceptors for NADPH cytochrome c reductase. This finding indicates that the lyase activity is inhibited by diversion of electron flow and, more specifically, that the point of interruption is at the reductase. Cytochrome c, itself, is a noncompetitive inhibitor of lyase activity when NADPH is substrate. Lyase and reductase activity were diminished to an equal extent by heating a microsomal suspension. An antibody to cytochrome e reductase was prepared after purification of the reductase from rat liver. This antibody caused a parallel and equal reduction of activity of the lyase and reductase. These data show that the reductase and lyase activities are closely linked and suggest that the reductase functions as an electron carrier for the reduction of P-450.