title: Pre-steady-state phosphorylation of the human red cell Ca2+-ATPase.
abstract: The pre-steady-state kinetics of phosphorylation of the Ca2+-ATPase by ATP was studied at 37 degrees C and in intact red cell membranes to approach physiological conditions. ATP and Ca2+ activate with K0.5 of 4.9 and 26.4 microM, respectively. Preincubation with Ca2+ did not change the K0.5 for ATP. Preincubation with ATP did not alter the initial velocity of phosphorylation suggesting that binding of ATP was not rate-limiting. Mg2+ added at the start of the reaction increased the initial rate of phosphorylation from 4 to 8 pmol/mg/s. With 30 microM Ca2+, the K0.5 for Mg2+ was 60 microM. Mg2+ and Ca2+ added together beforehand accelerated phosphorylation to 70 pmol/mg/s. Phosphorylation of calmodulin-bound membranes was the fastest (280 pmol/mg/s), and its time course showed a neat overshoot before steady state. The results suggest that either preincubation with Ca2+ plus Mg2+ or calmodulin accelerated phosphorylation shifting toward E1 the equilibrium between the E1 and E2 conformers of the enzyme. K+ had no effect on the initial rate of phosphorylation and lowered by 40% the steady-state level of phosphoenzyme in the absence of Mg2+. Phosphorylation is not rate-limiting for the overall reaction since its initial rate was always higher than ATPase activity. In the absence of K+, the turnover of the phosphoenzyme was 2000 min-1, which is close to the values for other transport ATPases.
authors.affiliation: Instituto de Química y Fisicoquímica Biológicas (Universidad de Buenos Aires-Consejo Nacional de Investigaciones Científicas y Técnicas), Facultad de Farmacia y Bioquímica, Argentina.
authors.forename: H P
authors.initials: HP
authors.lastname: Adamo
authors.forename: A F
authors.initials: AF
authors.lastname: Rega
authors.forename: P J
authors.initials: PJ
authors.lastname: Garrahan
mesh_terms: D002118:Calcium / Q000494:pharmacology; D000252:Calcium-Transporting ATPases / Q000097:blood*; D002147:Calmodulin / Q000494:pharmacology; D003198:Computer Simulation; D004912:Erythrocytes / Q000201:enzymology*; D006801:Humans; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008962:Models, Theoretical; D010766:Phosphorylation
chemical_list: D002147:Calmodulin; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium