Evidence for Random Distribution and Translational Movement of Cytochrome b5 in Endoplasmic Reticulum

Abstract Evidence is presented that electron transfer between different molecules of cytochrome b5 does not occur at an appreciable rate in microsomes. In view of this fact, the earlier observation that a large excess of functional amphipathic cytochrome b5 can be bound to microsomes suggests that cytochrome b5 and NADH-cytochrome b5 reductase are randomly distributed on the microsomal membrane. This model requires translational diffusion of these proteins within the membrane prior to interaction. The dependence of the rate of reduction of microsomal cytochrome b5 by NADH upon the concentration of cytochrome b5 and the effect of partial inactivation of microsomal NADH-cytochrome b5 reductase on the extent of cytochrome b5 reduction are both consistent with this model for the microsomal membrane. The evidence indicates that the interaction of the active sites of cytochrome b5 and reductase is not significantly altered by binding to the membrane and suggests that the hydrophilic portions of these proteins, although securely bound to the membrane by means of a hydrophobic peptide, may be capable of independent and significant motion in the aqueous phase.