ABSTRACT
This paper is a short review of the comparative biochemistry of the Na, K-ATPase and Ca2+ -ATPase of plasma membranes. The two ATPases share the same biphasic activation by ATP. Ca2+ - ATPase activation by ATP is strongly affected by calmodulin. The possibility of Mg2+ occlusion is proposed in connection with low-affinity activation by ATP. Both ATPase are activated by alkaline earth metal ions and display phosphatase activity toward p-nitrophenylphosphat for which Ca2 + - ATPase is strongly dependent on K + and regulated by calmodulin. The requirements for ligands of the phosphatase activity of both ATPase are strikingly similar except for the effect of calmodulin. Both ATPases are inhibited by vanadat and for both the effect of vanadate is modulated by Mg2+ and K + in the same way. These similarities indicate that, although Ca2 + - ATPase and Na, K-ATPase are different enzymes, their mechanisms of action may have more feactures in common than previously thought