A pK change of acidic residues contributes to cation countertransport in the Ca-ATPase of sarcoplasmic reticulum. Role of H+ in Ca(2+)-ATPase countertransport

Variable stoichiometric efficiency of Ca2+ and Sr2+ transport by the sarcoplasmic reticulum ATPase

In comparative experiments with Ca2+ ATPase in native sarcoplasmic reticulum vesicles and reconstituted proteoliposomes, we find that a variable stoichiometry of Ca2+ or Sr2+ transport per ATPase cycle is observed in the absence of passive leak through independent channels. The observed ratio is commonly lower than the optimal value of 2 and depends on the composition of the reaction mixture. In all cases, a progressive rise in the lumenal concentration of Ca2+ and Sr2+ is accompanied by a parallel reduction of coupling ratios. Significant ATPase activity remains even after asymptotic levels of Ca2+ accumulation are reached. This residual activity subsides if the Ca2+ concentration in the outer medium is reduced below activating levels (as it would following Ca2+ transients in muscle fibers). The reduction of stoichiometric coupling is explained with a reaction scheme, including a branched pathway for hydrolytic cleavage of phosphorylated intermediate before release of Ca2+ into the lumen of the vesicles. Flux through this pathway is favored when net lumenal Ca2+ dissociation from the phosphoenzyme is impeded and results in P(i) production accompanied by lumenal and medium Ca2+ exchange. Occurrence of reactions through branched pathways may have general implications for the stoichiometric efficiency of energy-transducing enzymes.

Characterisation of an intracellular Ca2+ pump in Dictyostelium

Calcium uptake by microsomal membranes from the cellular slime mould Dictyostelium discoideum was measured using Calcium Green-2 as a fluorescent probe of external free Ca2+ concentration. High-affinity Ca2+ uptake was found to be completely inhibited by low concentrations of vanadate, but not by thapsigargin, suggesting that the activity is mediated by a Ca(2+)-ATPase distinct from sarco(endo)plasmic reticulum type of higher animal cells. On sucrose density gradients, Ca2+ uptake distributes with vacuolar proton pump activity and part of the observed Ca2+ uptake is dependent on the pH gradient generated by the vacuolar-type H(+)-ATPase, indicating that the Ca2+ pump is located on both acidic and non-acidic vesicles, possibly derived from the H(+)-ATPase-rich contractile vacuole complex.