Potassium-activated phosphatase from human red blood cells : The Effects of Adenosine Triphosphate

The behavior of the red cell membrane K(+)-activated phosphatase is significantly altered by ATP. When ATP is added, the apparent affinity of the enzyme for its substrate and for K(+) is lowered, whereas its sensitivity to ouabain is increased. Under these conditions, addition of Na(+) raises the apparent affinity of the enzyme for K(+) to values well above those found in the absence of Na(+) and ATP. The effect of Na(+) is blocked by hydroxylamine and oligomycin. Low concentrations of Ca(++), Sr(++) or Ba(++), which have little effect in the absence of ATP, induce large increases in the K(+)-dependent phosphatase activity in the presence of ATP. This effect is associated with the loss of ouabain sensitivity of the phosphatase. The velocity vs. divalent cation concentration curves of the K(+)-dependent phosphatase and the (Na(+)+K(+))-independent ATPase activities are very similar. The effects of ATP seem to be specific for this nucleotide and are exerted at concentrations similar to those normally found in red cells. They may therefore be relevant to the proposed physiological role of the cell membrane phosphatase.