CA2-RETICULUM ATPASE RANDOMIZES THE 2 TRANSPORTED IONS( TRANSLOCATIONACROSS SARCOPLASMIC)

Cytoplasmic Ca2+ dissociation is sequential, and the Ca2+ ions bound t o the nonphosphorylated ATPase are commonly represented as superimpose d on each other, so that the superficial Ca2+ is freely exchangeable f rom the cytoplasm, whereas the deeper Ca2+ is not. Under conditions wh ere ADP-sensitive phosphoenzyme accumulates (leaky vesicles, 5 degrees C, pH 8, 300 mM K+), luminal Ca2+ dissociation is sequential as well, so that the representation of two superimposed Ca2+ ions still holds on the phosphoenzyme, with the superficial Ca2+ facing the lumen freel y exchangeable and the deeper Ca2+ blocked by the superficial Ca2(+). Under the same conditions, we have investigated whether a prebuilt Ca2 + order is maintained during membrane translocation, Starting from a p rebuilt order on the cytoplasmic side, we showed that the Ca2+ ions ca nnot be identified after translocation to the luminal side, The same r esult was obtained starting from a prebuilt order on the luminal side and following the luminal to cytoplasmic translocation, We conclude th at the two Ca2+ ions are mixed during ATP-induced phosphorylation as w ell as during ADP-induced dephosphorylation.