Evidence Obtained by Cathepsin Digestion of Microsomes for the Assembly of Cytochrome b5 and Its Reductase in the Membrane

Abstract

Treatment of rat liver microsomes with cathepsin D [EC 3.4. 4. 23] resulted in the selective solubilization of NADH-cytochrome b₅ reductase [EC 1.6. 2. 2. ], leaving other electron transferring components such as cytochrome b₅, NADPH-cytochrome c reductase [EC 1.6.2.4] and cytochrome P-450 still attached to the membrane. The rate and the extent of reduction of membrane-bound cytochrome b₅ by NADH were studied with digested microsomal samples containing different amounts of the cytochrome b₅ reductase. The analysis of the reduction kinetics of the cytochrome b₅ suggested the presence of the assemblies of cytochrome b₅ and its reductase in the original microsomal membrane. An assembly seems to contain about five molecules of NADH cytochrome b₅ reductase and about fifty molecules of cytochrome b₅ gathered togather in the membrane.