Interaction of [formula omitted] and Ca 2+ with human erythrocyte membrane ATPase

Abstract

Human erythrocyte membranes were treated with N- ethylmaleimide and then assayed for cation-sensitive components of ATPase activity. Inhibition of Na +-activated ATPase and (Na +-K +)-activated ATPase (ATP phosphohydrolase, EC 3.6.1.3), observed following N- ethylmaleimide treatment, was abolished when ethylene glycol bis-(β-aminoethyl)-N,N- tetraacetate (EGTA) was included in the assay system. N- Ethylmaleimide pretreatment reduced Mg 2+-activated ATPase activity both in the absence and presence of EGTA. The Mg 2+-dependent components of [ 14C]ADP-ATP exchange and membrane phosphorylation using [ γ- 32 P] ATP were reduced following N- ethylmaleimide pretreatment whereas the Na +-stimulated components of these activities were unaffected. At very low ATP concentration, chelation of endogenous Ca 2+ increases the sensitivity of the ATPase system to stimulation by low concentrations of Na +. In contrast a Ca 2+-requirement at a Na +-dependent step of (Na +-K +) activated ATPase reaction sequence was observed at higher ATP concentration. These results suggest both inhibitory and activating effects of Ca 2+ on alkali cation-sensitive ATPase.