The purification and properties of microsomal cytochrome reductase.

In earlier work (1) a DPNH’-specific microsomal cytochrome reductase, liberated by alcohol extraction and partially purified, was identified in liver microsome fractions from rats and rabbits. Microsomal cytochrome, but not cytochrome c, was found to act as electron acceptor in the oxidation of DPNH catalyzed by the enzyme. Through a rapid cytochrome to cytochrome reaction, cytochrome c was reduced in this system when the microsomal cytochrome was added as an intermediate electron carrier. Although flavin was found in the preparations, the large amount of contaminating protein made it impossible to determine conclusively whether the flavin was a prosthetic group on the reductase. The present report describes the isolation of this enzyme as an essentially homogeneous flavoprotein from calf liver microsomes by a new method. Measurements of the molecular weight and physical properties, the characterization of the coenzyme and other active groups, analysis for possible metal cofactors, and studies of the general enzymatic properties of the reductase system are presented. The yield of microsomal cytochrome reductase establishes this enzyme as a major part of the so called DPNH cytochrome c reductase in microsomes.