Immunochemical studies on the large polypeptide of electrophorus electroplax (Na+ + K+)-ATPase

Antibodies against Lubrol-solubilized Electrophorus electroplax (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) and its 96 000-dalton polypeptide (P96) were raised in rabbits. The P96 antibody does not cross react with the (Na+ + K+)-ATPase from mammalian species and tissues, but it cross reacts with the (Na+ + K+)-ATPase from both Electrophorus electroplax and brain. The combination of enzyme with anti-P96 is found to inhibit phosphoryl enzyme formation to the same extent that it inhibits enzyme activity. The rate of K+-sensitive dephosphorylation of phosphoryl enzyme appears to be unchanged. These are also found to be true with the antibody against the whole enzyme. Upon tryptic digestion of the enzyme-anti-P96 complex only the large polypeptide of the enzyme is protected. In the case of enzyme-anti-Lubrol-solubilized enzyme complex, both the large and small polypeptides are protected, whereas preimmune sera are without any protecting effect. The data indicate that the phosphoryl acceptor polypeptide and the Lubrol-solubilized electroplax (Na+ + K+)-ATPase from which the polypeptide is derived are phylogenetically distinct from those of the mammalian (Na+ + K+)-ATPases. The selective tryptic resistance of the enzyme-anti-P96 complex indicates that the two polypeptides are spatially well separated, possibly on opposite sides of the membrane.