The kinetics of the hydrolysis of ATP by a rat-brain lipoprotein fraction have been investigated. The activity is greatly increased in the presence of Na+ and K+, but reaches a maximum value and then suffers a diminution in the presence of an excess of either ion. Kinetic evidence indicates that these ions combine with the enzyme protein according to Michaelis-Menten kinetics, that 2 Na+ are involved in the activation, and that the interaction of Na+ and K+ at each site is of a competitive nature. Inhibition of the (Na+K+)-stimulated ATPase by ouabain, appears to result from a competition between ouabain and both K+ and Na+ while addition of oligomycin leads to kinetics which suggest competition of oligomycin with Na+. NH4+ may substitute for K+ and, when present in excess, may also give rise to inhibitory effects.
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It is a pleasure for one of us (P.G.S.) to thank the Chicago Medical School for an appointment as a Visiting Professor which made possible this cooperation with the Division of Metabolic Research of the Chicago Medical School. It is also a pleasure to thank the National Cancer Institute of Canada for support of the work in Montreal and to thank the U.S. Public Health Service (Grant No. AM 7226), the Life Insurance Medical Research Fund, and the Otho S.A. Sprague Memorial Foundation of Chicago for support of the work carried out in Chicago.