Kinetic studies on inhibition of dog heart Na+,K+-dependent ATPase by K+,Mg2+-aspartate: comparison with ouabain.

Both K+,Mg+-dependent D,L-aspartate and D,L-aspartic acid are noncompetitive inhibitors of dog heart Na+,K+-ATPase acting at the overlapping site as does ouabain. For the Na+,K+-ATPase, the salient effects of K+,Mg+-D,L-aspartate and/or D,L-asartic acid were: 1) decrease in maximal velocity (V) for ATP as substrate with unchanged Km; 2) for sodium as an allosteric modifier of Na+,K+-ATPase activity, a decrease in V without any alteration in n as measure of cooperativity between activating sites; 3) for potassium, a decrease in V and n as well as an increase in K0.5 values. Thus, for enzyme activity in the presence of sodium and ATP, a high affinity to potassium was reduced by D,L-aspartic acid.