Abstract
A quantitative electron microscopic method for the demonstration of (Na+ + K+)-adenosine triphosphatase (ATPase) in human erythrocyte ghost membranes is described which differs from that employed previously by introducing several modifications which greatly reduce the possibility that variations in section thickness or contamination or sublimation in the electron beam could effect the result. This procedure makes use of the greater electron density of Cs+ rather than K+ ions as an activator of (Na+ + K+)-ATPase, thus avoiding the lack of specificity of other histochemical or cytochemical procedures. The transient formation of an ion-carrier state during the hydrolysis of ATP by erythrocyte ghost membrane (Na+ + K+)-ATPase was shown by a significant increase in membrane density when Cs+ ions accumulate at the membrane. This accumulation occurs only in the presence of ouabain. The increase in membrane specific gravity was a generalized phenomenon and was not localized at discrete areas of the membrane, suggesting that (Na+ + K+)-ATPase "pumping sites" are distributed uniformly throughout the total membrane structure.