Dephosphorylation kinetics of pig kidney Na+,K+-ATPase

The kinetics of K+-stimulated dephosphorylation of the Na+,K+-ATPase were investigated at pH 7.4, 24 degrees C, and an ATP concentration of 1.0 mM via the stopped-flow technique using the fluorescent label RH421. Two different mixing procedures were used: (a) premixing with ATP to allow phosphorylation to go to completion, followed by mixing with KCl; and (b) simultaneous mixing with ATP and KCl. Using mixing procedure (a), the dephosphorylation rate constant of enzyme complexed with K+ ions could be determined directly to be less than or equal to 366 s(-1) and the rate constant for spontaneous dephosphorylation (without K+) less than or equal to 60 s(-1). The K+ concentration dependence of the observed reciprocal time constant showed half-saturation at a K+ concentration of 2.4-2.6 mM with positive cooperativity involved in the occupation of the K+ binding sites on the E2P conformation of the enzyme. Using mixing procedure (b), it was found that at saturating K+ concentrations the dephosphorylation of the enzyme is rate-limited by its phosphorylation, which occurs with a rate constant of approximately 190 s(-1) (1). These results show that all reactions occurring after phosphorylation and prior to dephosphorylation, i.e., the E1P to E2P conformational transition as well as Na+ release and K+ binding steps, must be fast (>190 s(-1)).