Studies on red-cell ghost ATPase systems: Properties of a (Mg2+ + Ca2+-dependent ATPase

• 1.

  1. In the presence of Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ have an activating effect on the ATPase activity of human red-cell ghosts, the greatest activity being obtained with Sr²⁺ as the activator.

• 2.

  2. The presence of Ca²⁺ and Sr²⁺ tends to prevent the activation by Na⁺ and K⁺.

• 3.

  3. The activating effects of Mg²⁺ and Ca²⁺ appear to differ in their point of attack; Sr²⁺ and Ba²⁺ seem to be bound by the same sites as Ca²⁺.

• 4.

  4. There is an optimal ratio between the concentrations of Mg²⁺ and Ca²⁺ (or Sr²); excess Ca²⁺ (or Sr²⁺) has an inhibiting effect, presumably by interfering with Mg²⁺ activation. Excess Mg²⁺ may also interfere with Ca²⁺ (or Sr²⁺) activation.

• 5.

  5. The optimal pH is somewhat higher for the (Na⁺ + K⁺)-dependen activity than for the (Mg²⁺ + Ca²⁺)-dependent activity and the curves of the activity as a function of pH have different shapes.

• 6.

  6. The (Mg²⁺ + Ca²⁺)-dependent activity is inhibited by Salygran, Atebrin and more specifically by sodium Amytal and 2,4-dinitrophenol. In the presence of Mg²⁺ alone, the ATPase activity is inhibited by Atebrin, Salygran and, more specifically, by guanidin. The (Mg²⁺ + Na⁺ + K⁺)-dependent component is inhibited by Atebrin and Salyrgan but is insenstivie to guanidin and 2,4-dinitrophenol.

• 7.

  7. The properties of the (Mg² + Ca²⁺)-dependent enzymic system are analogous to those of myofibrillar ATPase; thus, this system may be identified with the actomyosin-like proteins described in red-cell ghosts by Ohnishi.

• 8.

  8. In the presence of 2,4-dinitrophenol in amounts sufficient to inhibit the (Mg²⁺ + Ca²⁺)-dependent ATPase, one observes in intact cells a marked increase in the passive permeability to Na⁺ but no effect on active transport. This suggests that contractile proteins of red cells may be involved in the regulation of passive permeability to Na⁺.