The effect of lithium and potassium on the transient state kinetics of the (Ca + Mg)-ATPase of cardiac sarcoplasmic reticulum.

KCl or LiCl, when added in 100 mM concentrations to cardiac sarcoplasmic reticulum incubated at 17 degrees C with 5 micron [gamma-32P]ATP, 1 mM MgCl2, and 9.1 micron M Ca2+, increased the apparent phosphorylation rate constant from 14.5 s-1 to 23.8 s-1 (100 mM LiCl) or to 44.1 s-1 (100 mM KCl). These same monovalent cations also increased the apparent rate constant for the hydrolysis of the phosphorylated sarcoplasmic reticulum from 0.51 s-1 to 1.12 s-1 (100 mM LiCl) or to 1.71 s-1 (100 mM KCl). Although there was a small burst in Pi production, rate constant of 0.97 s-1, when 100 mM KCl was added, the burst when LiCl or no monovalent cation was added was either nonexistent or so small as to make its detection unreliable. KCl thus appears to induce an intermediate which is either nonexistent when omitted or in such low concentration as not to be readily detected.