title: Pre-steady-state kinetic study of the effects of K+ on the partial reactions of the catalytic cycle of the plasma membrane Ca(2+)-ATPase.
abstract: The effects of 100 mM K+ on the partial reactions that take place during ATP hydrolysis on the calcium ion-dependent ATPase from plasma membrane (PM-Ca(2+)-ATPase) were studied at 37 degrees C on fragmented intact membranes from pig red cells by means of a rapid chemical quenching technique. At 10 microM [gamma-32P]ATP plus non-limiting concentrations of Ca2+ and Mg2+, K+ increased the k(app) of formation by 140% to 84 11 s-1 and the steady-state level of phosphoenzyme (EP) by 25% to 3.4 0.17 pmol/mg of protein. If added together with [gamma-32P]ATP at the beginning of phosphorylation, K+ was much less effective than if added earlier, indicating that it did not act on the phosphorylation reaction. Measurements of the E2 --> E1 transition by phosphorylation showed that in medium with Ca2+ and Mg2+, K+ increased the k(app) of the transition by 55% to 14 3 s-1 and the apparent concentration of E1 by 45%, suggesting that this may be the cause of the increased rate of phosphorylation observed in enzyme preincubated with K+. The presence of K+ did not change the slow decay of EP without Mg2+ but activated the decay of EP made with Mg2+, increasing its k(app) by 60% to 91 12 s-1. In contrast with observations made during phosphorylation, if added at the beginning of dephosphorylation K+ was fully effective in favouring decomposition of EP made in medium containing no K+. In the presence of either 3mM ATP or 3 mM ATP plus calmodulin, which activate hydrolysis of CaE2P, the effect of K+ on dephosphorylation was conserved. Because the sites for K+ are intracellular and the concentration of K+ in normal red cells is above 100 mM, the effects described here must be taken into account to describe the catalytic cycle of the PM-Ca(2+)-ATPase under physiological conditions.
authors.affiliation: Instituto de Química y Fisicoquímica Biológicas, Facultad de Farmacia y Bioquímica, Buenos Aires, Argentina.
authors.forename: C J
authors.initials: CJ
authors.lastname: Herscher
authors.forename: A F
authors.initials: AF
authors.lastname: Rega
authors.forename: H P
authors.initials: HP
authors.lastname: Adamo
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D000252:Calcium-Transporting ATPases / Q000737:chemistry / Q000378:metabolism*; D002384:Catalysis; D004910:Erythrocyte Membrane / Q000187:drug effects* / Q000201:enzymology*; D066298:In Vitro Techniques; D007700:Kinetics; D010766:Phosphorylation; D011188:Potassium / Q000494:pharmacology*; D013552:Swine
chemical_list: D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D011188:Potassium