The reactive sulfhydryl groups of microsomal cytochrome reductase.

The results of previous work have indicated (2, 3) that one sulfhydryl group of microsomal cytochrome reductase is essential for the interaction of nucleotides with this enzyme. These results, however, do not rule out the possibility that there are other reactive sulfhydryl groups on the native enzyme and that they are involved directly or indirectly in nucleotide-enzyme interactions. In the experiments described below, sulfhydryl group reagents have been used to characterize the reactive sulfhydryl groups of the native enzyme. It will be shown that each sulfhydryl group can be distinguished from the others by at least one property. On the basis of these properties, N-ethyl maleimide derivatives of microsomal cytochrome reductase, containing between one and two reactive sulfhydryl groups per molecule of protein, have been prepared. These derivatives catalyze all of the reactions of microsomal cytochrome reductase thus establishing that only one specific sulfhydryl group is involved in nucleotide interactions with the enzyme.