Oligomycin induces occlusion of Na+ in membrane-bound Na,K-ATPase. Here it is shown that Na,K-ATPase from pig kidney or shark rectal gland solubilized in the nonionic detergent C12E8 is capable of occluding Na+ in the presence of oligomycin. The apparent affinity for Na+ is reduced for both enzymes upon solubilization, and there is an increase in the sigmoidicity of binding curves, which indicates a change in the cooperativity between the occluded ions. A high detergent/protein ratio leads to a decreased occlusion capacity. De-occlusion of Na+ by addition of K+ is slow for solubilized Na,K-ATPase, with a rate constant of about 0.1 s-1 at 6 degrees C. Stopped-flow fluorescence experiments with 6-carboxyeosin, which can be used to monitor the E1Na-form in detergent solution, show that the K(+)-induced de-occlusion of Na+ correlates well with the fluorescence decrease which follows the transition from the E1Na-form to the E2-form. There is a marked increase in the rate of fluorescence change at high detergent/protein ratios, indicating that the properties of solubilized enzyme are subject to modification by detergent in other respects than mere solubilization of the membrane-bound enzyme. The temperature dependence of the rate of de-occlusion in the range 2 degrees C to 12 degrees C is changed slightly upon solubilization, with activation energies in the range 20-23 kcal/mol for membrane-bound enzyme, increasing to 26-30 kcal/mol for solubilized enzyme. Titrations of the rate of transition from E1Na to E2K with oligomycin can be interpreted in a model with oligomycin having an apparent dissociation constant of about 2.5 microM for C12E8-solubilized shark Na,K-ATPase and 0.2 microM for solubilized pig kidney Na,K-ATPase.