Specific effects of spermine on Na+,K+-adenosine triphosphatase

Specific effects of spermine on Na+,K+-ATPase were observed using an enzyme partially purified from rabbit kidney microsomes by extraction with deoxycholate. 1. Spermine competed with K+ for K+-dependent, ouabain-sensitive nitrophenylphosphatase. The K1 for spermine was 0.075 mm in the presence of 1 mM Mg2+ and 5 mM p-nitrophenylphosphate at pH 7.5. 2. spermine activated Na+,K+-ATPase over limited concentration ranges of K+ and Na+ in the presence of 0.05 mM ATP. The spermine concentration required for half maximal activation was 0.055 mM in the presence of 1 mM K+, 10 mM Na+, 1 mM Mg2+, and 0.05 mM ATP. 3. The activation of Na+,K4-ATPase was not due to substitution of spermine for K+, Na+, or Mg2+. 4. When the concentration of K+ or Na+ was extremely low, or in excess, spermine did not activate Na+,K+-ATPase, but inhibited it slightly. 5. Plots of 1/v vs. 1/[ATP] at various concentrations of spermine showed that spermine decreased the Km for ATP without changing the Vmax. 6. Plots of 1/v vs. 1/[ATP] at concentrations of K+ from 0.05 mM to 0.5 mM showed that K+ increased the Km for ATP with increase in the Vmax in the presence of 0.2 mM spermine similarly to that in the absence of spermine. The contradictory effects of spermine on this enzyme system suggest that the K+-dependent monophosphatase activity does not reflect the second half (the dephosphorylation step) of the Na+,K+-ATPase catalytic cycle.