On the Action of Strophanthin G

The inhibitory effect of ouabain on Na- and K-activated ATP-ase of rat brain lipoprotein is reduced when the incubation temperature is lowered. The concentration of K+required for maximal ATP-ase activity is also reduced at lower temperatures of incubation. The apparent increase in affinity for K+by the enzyme system at lower incubation temperatures could be responsible for the change in the effectiveness of ouabain. Increase in enzyme protein concentration also reduces the inhibition produced by a given concentration of ouabain, which appears to be due to binding of ouabain by the lipoprotein. From the titration of enzyme by ouabain it is possible to estimate that 5 × 1014molecules of ouabain are taken up per milligram of lipoprotein.