Two classes of site for ATP in the Ca2+-ATPase from human red cell membranes

(1) The response of the Ca2+-ATPase activity from human red cell membranes to ATP concentrations can be represented by the sum of two Michaelis-like curves: one with a Km of 2.5 micrometer and the other with a Km of 145 micrometer. (2) The maximum Ca2+-ATPase activity elicited by occupation of the site with lower Km represents about 10% of the activity attainable at non-limiting ATP concentrations. (3) 30--50% of the Ca2+-ATPase activity with lower Km remains in the absence of Mg2+ . Mg2+ increases V and the maximum effect of Ca2+, having no effect on the apparent affinities for ATP and Ca2+. (4) The large increase in Ca2+-ATPase activity which results from the occupation of the site with higher Km only takes place when Mg2+ is present. (5) Results are compatible with the idea that the Ca2+-ATPase from human red cell membranes has two classes of site for ATP binding, both of which are occupied when the enzyme catalyzes the hydrolysis of ATP at maximum rate. (6) The properties of the high affinity site suggest that this is the catalytic site of the Ca2+-ATPase. It is proposed that binding of ATP at the low affinity site regulates the turnover of the system.