Mg2+-dependent, (Na+ + K+)-stimulated ATPase of human platelets: Properties and inhibition by ADP

The preparation and properties of a Mg²⁺-dependent, (Na⁺ + K⁺)-stimulated ATPase from platelets are described. The enzym system require Mg²⁺ for activity, is stimulated maximally by the presence of Na⁺ and K⁺ in a ratio of 11.5/1, has a pH optimum 7.30–7.40, and is half-maximally inhibited by ouabain at a concentration of 0.15 μM. The K_m for ATP is approx. 0.4 mM. Enzyme activity is localized in both the “membrane” and “membrane-granule” subcellular fractions.Mg²⁺-dependent, (Na⁺ + K⁺)-stimulated ATPase activity is inhibited by ADP (which induces platelet aggregation) and by P_i. The inhibition by ADP is apparently not competitive with respect to ATP. Mg²⁺-dependent, K⁺-stimulated p-nitrophenyl role=presentation style=font-size: 90%; display: inline-block; position: relative;>$\text{p-}\text{nitrophenyl}$$\text{p-}\text{nitrophenyl}$ phosphatase activity (a model for the externally-oriented K⁺-phosphatase portion of Mg²⁺-dependent, (Na⁺ + K⁺)-stimulated ATPase) is also inhibited by ADP. GDP, which does not induce aggregation of platelets, has no effect on the Mg²⁺-dependent, (Na⁺ + K⁺)-stimulated ATPase activity.It is postulated that ADP interacts with the external platelet membrane surface to inhibit Mg²⁺-dependent, (Na⁺ + K⁺)-stimulated ATPase activity with a resultant decrease in cation transport and membrane potential. This phenomenon may play a role in platelet aggregation.