Observations on the protein activator of erythrocyte membrane (Ca2+ + Mg2+)ATPase.

• 1.1. SDS-polyacrylamide gel electrophoresis separated the protein activator of pig erythrocyte membrane (Ca²⁺ + Mg²⁺)ATPase into two protein bands, both being active in stimulating the enzyme. Disc gel electrophoresis (12% acrylamide, 0.6% N,N′-methylenebisacrylamide) gave four active protein bands.
• 2.2. The activator from pig erythrocytes contains a high content of glutamic acid (28%) and aspartic acid (20%) but relatively low levels of lysine (6%) and arginine (2%).
• 3.3. Optimal pH for (Ca²⁺ + Mg²⁺)ATPase activation by the activator is 7.7. Sodium but not potassium enhances the activation.
• 4.4. A minimum of 680,000 activator molecules were estimated to be associated with sites on the membrane of one erythrocyte.
• 5.5. The activator from pig red cells stimulated ATP dependent calcium uptake into inside-out vesicles prepared from human erythrocytes at a calcium concentration of 10 μM or 100 μM.
• 6.6. Protein activators of pig erythrocyte membrane (Ca²⁺ + Mg²⁺)ATPase were identified in red cells of various vertebrates and in perinatal and neonatal rat erythrocytes.