Use of GTP to distinguish calcium transporting ATPase activity from other calcium dependent nucleotide phosphatases in human placental basal plasma membrane.

In many cells other than the erythrocyte, the relationship between ATP dependent calcium transport and calcium dependent ATP hydrolysis is complex. The characteristics of ATP hydrolysis often differ from those of calcium transport. Demonstration of a specific transport ATPase is complicated by heterogeneity and high background activity in the presence of magnesium. In basal plasma membrane of human placental syncytiotrophoblast, the addition of 5 mM GTP greatly reduces the background release of 32Pi from 0.1 mM [gamma, 32P]-ATP. The addition of GTP permits measurement of high affinity calcium dependent ATPase under conditions which support calcium uptake. GTP does not affect the velocity of calcium uptake, and in its presence the calcium and magnesium concentration dependence of calcium uptake and calcium dependent ATPase are similar.