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From the
1 From the Laboratory of Neurochemistry, National Institute of Neurological Diseases and Blindness, National Institutes of Health, Bethesda, Maryland 20014
Microsomes prepared from electric organ of Electrophorus electricus contain, in addition to a sodium- and potassium-activated adenosine triphosphatase, two different ATP-ADP transphosphorylases; one requires only magnesium, while the other requires Mg++ + Na+. The Mg++-activated nucleotide exchange is nonspecific with respect to substrates and is probably unrelated to the highly specific Na+-K+-ATPase. The (Mg++ + Na+)-activated exchange is highly specific with respect to Na+ and the adenine nucleotides and is probably a component of the Na+-K+-ATPase. However, the Na+-activated exchange may be deomonstrated only at low Mg++ concentrations relative to the optimum for the ATPase. Calcium, K+, ouabain, and p-chloromercuribenzoate inhibit both the Na+-activated exchange and the ATPase. Oligomycin and incubation at pH 9 inhibit the ATPase with little effect on the exchange.
It is concluded that the initial step of the Na+-K+-ATPase reaction is a reversible phosphorylation of the enzyme which requires both Na+ and Mg++ as activators. The nonspecific nucleotide exchange is considered to be irrelevant to the Na+-K+-ATPase.
Submitted on September 16, 1965
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