Potassium-potassium exchange as part of the over-all reaction mechanism of the sodium pump of the human red blood cell.

When the efflux components of the Na-K exchange and K-K exchange are measured under identical conditions, the apparent K 1/2 (the concentration of K at which the velocity is half-maximal) for external K of the two processes differ. The discrepancy diminishes when the measurements are made in solutions containing low concentrations of Na, and in these solutions the uncoupled Na efflux is also partially suppressed. It is possible to demonstrate an uncoupled K efflux into solutions free of Na and K. This uncoupled efflux is also partially inhibited by low concentrations of external Na. At high enough concentrations, intracellular Na completely inhibits the K-K exchange. Inhibition of the K-K exchange by cell Na is competitive with cell K, and inhibition of the Na-K exchange by cell K is competitive with cell Na. In each case the characteristics of the competition suggest that both ions competitively interact with sites on the same enzyme form. The Albers-Post model of the Na-K pump reaction mechanism, modified to account for the uncoupled Na and K efflux, accounts in detail for these observations. If the K-K exchange is part of the over-all Na-K exchange, as indicated by the findings, pump models in which Na must add to the pump at the inside before K is released are excluded.