THE ROLE OF SODIUM IONS IN THE ACTIVATION OF ELECTROPHORUS ELECTRIC ORGAN ADENOSINE TRIPHOSPHATASE

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Abstract
Labeling of electrophorus electric organ Na+ -ATPase particles by ATP32 is stimulated by Mg++. The sites available for phosphorylation are increased severalfold when Na+ is also present. K+ decreases labeling and Li+ is without effect. Brief peptic digestion of "Mg++ sites" releases 3 major radioactive fragments, in addition to orthophosphate, which migrate anodally at pH 3.5. The electrophoretic pattern from "Mg++ + Na sites" contains an additional slowly migrating cathodal component which readily breaks down to release orthophosphate. Since Mg++ , but not Na+ , activates the ADP-ATP exchange reaction, Na+ sites are evidently phosphorylated secondarily to the Mg++ sites. These observations suggest that Na transport may be a consequence of oriented transphosphorylations across the cell membrane through an environment which selects for Na+ as the counterion.