Human red blood cells rapidly convert extracellular ferricyanide into extracellular ferrocyanide. The reaction is enhanced by the addition of a substrate, adenosine. This increase of the rate of reaction is abolished by iodoacetate. The results indicate there is a flow of electrons across the membrane of metabolizing red blood cells. The reduction of extracellular ferricyanide is accompanied by the formation of intracellular ATP. The effect of an uncoupler and of inhibitors of oxidative phosphorylation on this reaction was studied under conditions where the natural rate of ATP synthesis was slightly reduced by 10-4 moles/liter iodoacetate. ATP formation was found to be inhibited by DNP, cyanide, and, to a lesser extent, by azide. Amytal is ineffective. Ferrocyanide enhances ATP breakdown. The action of DNP requires the presence of the cell membrane. It can probably not be related to a stimulation of the membrane ATPase of Laris and Letchworth, nor can it be explained on the basis of Mitchell's chemiosmotic hypothesis by effects on the passive permeability of the erythrocyte membrane to H+ or alkali ions. In contrast to methylene blue and other oxidants, ferricyanide does not stimulate oxygen consumption in adult red blood cells. © 1969 Springer-Verlag New York Inc.
