Proton ejection as a major feature of the Ca(2+)-pump.
H+ ejection and Ca2+ uptake promoted by the sarcoplasmic reticulum (SR) Ca(2+)-pump are similarly stimulated by millimolar Mg2+. This cannot be assigned to enhanced Ca2+ uptake and H+ displacement from internal metal binding sites since: (1) loading SR vesicles with high Mg2+ concentrations does not impair H+ ejection; (2) loading SR vesicles with Mn2+ does not depress H+ ejection occurring during Mn2+ uptake; (3) H+ ejection occurs even when Ca2+ accumulation inside the vesicles is prevented with Ca2+ ionophores. It is concluded that the Ca(2+)-pump promotes an active Ca2+/H+ countertransport stimulated by Mg2+. Finally, a mechanism for Ca2+ translocation is proposed in basic physico-chemical terms.
da Costa
A G
AG
Centro de Biologia Celular e Departamento de Bioquímica, Coimbra, Portugal.
Madeira
V M
VM
D000818:Animals; D002021:Buffers; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D007832:Lasalocid / Q000494:pharmacology; D008274:Magnesium / Q000494:pharmacology; D008345:Manganese / Q000494:pharmacology; D011522:Protons; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D013997:Time Factors
D002021:Buffers; D011522:Protons; D000001:Calcimycin; D008345:Manganese; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium; D007832:Lasalocid