Potassium activated phosphatase from human red blood cells. The mechanism of potassium activation.

1. The kinetic behaviour of the K-activated phosphatase in human red blood cell membranes has been investigated. The concentration of Mg required to give optimal activation is independent of substrate and K concentration, suggesting that Mg combines with the enzyme at a site that is independent of and non-interacting with the substrate and K sites.2. The effects of K are competitively antagonized by Na. Ouabain in suitable concentrations selectively abolishes the activating effect of K.3. Comparison between the hydrolysis of acetylphosphate by intact red cells and by fragmented ghosts suggests that the active site for the substrate is only accessible at the internal surface of the cell membrane.4. The plot of the total rate of p-nitrophenylphosphate hydrolysis versus substrate concentration can be fitted at any K concentration by a rectangular hyperbola. The effect on the total rate of increasing K concentration is exerted mainly on the apparent affinity for the substrate, which increases about 5 times as K goes from 0 to 55 mM. There is also a much smaller increase in the maximum velocity (about 1.3 times) for the same range of K concentrations.5. If the difference between the activity in the absence and in the presence of K is plotted as a function of substrate concentration, the curves obtained are no longer hyperbolic but pass through a maximum and then tend to a lower value.6. This kinetic behaviour can be explained much more easily by assuming that a single enzyme is responsible for the hydrolysis of the substrate in the presence and in the absence of K. A simple kinetic model based on this assumption was developed and when experimentally determined constants were fitted into the equations that predict its behaviour a reasonably good agreement between theory and experiment was obtained.7. In the ;single enzyme' model inhibitors that selectively abolish the K-dependent activity would act by blocking the combination of the enzyme with K. A simple treatment based on this idea was developed for the case of Na and its predictions were fulfilled by the experimental results.8. In the ;single enzyme' model the K-coupled hydrolysis is always larger than the difference between the rates in the presence and in the absence of K, and when K concentration is non-limiting the K-coupled rate is equal to the total rate.