Comparative studies on the ATPase-binding sites in Ca2+-ATPase and (Na+ + K+)-ATPase by the use of ATP-analogues.

The effects of ATP-analogues on Ca2+-ATPase and (Na++ K+)-ATPase have been studied. The participation of sulfhydryl groups in the recognition of ATP by both transport ATPases is indicated by the fact, that the disulfide of thioinosine triphosphate inactivates both enzymes. The reactivity of rapidly and slowly reacting sulfhydryl groups in the ATP binding sites of both enzymes is altered by the presence of transport substrates. At lease in (Na+ + K+)-ATPase Na+ and Mg2+ appear to alter the structure of the ATP binding site, which conclusion is fortified by the fact, that the photoinactivation of the enzyme by 3'-O-[3-(2-nitro-4-azidophenyl)-propionyl]-ATP needs Mg+. Chromium(III)ATP, a MgATP analogue, inactivated both transport ATPases by the formation of a stable chromo-phosphointermediate. In the case of Ca2+-ATPase this was concomited by the occlusion of Ca2+ in a stable form. No occlusion of Na+ was observable so far in the (Na+ + K+)-ATPase. Contrary to the expectation of the Albers-Post-scheme the hydrolysis of the phosphointermediate formed from chromium(III)ATP was protected by K+, but activated by high concentrations of Na+. Consequently, despite of the inhibition of (Na+ + K+)-ATPase activity, chromium(III)-ATP supported the Na+ - Na+-exchange reaction in everted red blood cells.