Specific effects of spermine on ouabain-sensitive and potassium-dependent phosphatase activity of kidney plasma membranes. Specificity of the potassium sites.

Specific inhibition of ouabain-sensitive and K+-dependent p-nitrophenyl-phosphatase activity of rabbit kidney plasma membranes by spermine (N,N'-bis(3-aminopropyl)-1,4-butanediamine) was characterized kinetically. 1. Inhibition by spermine was competitive with K+. The Ki for spermine was 31 micronM in the presence of 1 mM Mg2+. 2. Excess Mg2+ inhibited the ouabain-sensitive phosphatase activity in competition with K+. The Ki for Mg2+ was 2.6 mM. 3. Increasing Mg2+ concentrations reduced the spermine inhibition. This could be observed at Mg2+ concentrations higher than that of K+. 4. In the absence of inhibition by Mg2+, spermine was noncompetitive with Mg2+ which was essential for the ouabain-sensitive phosphatase activity. This could be observed at Mg2+ concentrations lower than that of K+. 5. Although Ca2+ was a strong inhibitor of the ouabain-sensitive phosphatase activity in the presence of K+, it produced a small stimulation of the activity in the absence of K+. Approximately 0.1 mM Ca2+ gave the maximum stimulation. 6. The observed Ca2+- and Mg2+-dependent phosphatase activity was inhibited strongly by ouabain and by spermine. The half-maximal inhibition concentrations of ouabain and spermine were 0.1 and 63 micronM, respectively. It is likely that Mg2+, Ca2+ and spermine bind to the same site as does K+.