Calcium translocation in liposome systems modeled on the mitochondrial inner membrane.
Ca2+ uptake by liposomes consisting of phosphatidylcholine (PC) and cardiolipin (CL) has recently been reported (Smaal, E.B. et al. (1987) Biochim. Biophys. Acta 897, 191-196). In eukaryotic cells, CL is localized exclusively in the inner mitochondrial membrane, where it occurs in the presence of equimolar amounts of PC and phosphatidyl-ethanolamine (PE). We have therefore re-examined CL-mediated Ca2+ translocation in liposomes of more nearly physiological composition, i.e., PC/PE/CL (2:2:1 and 4:4:1, mol/mol). In addition, the effect on Ca2+ uptake of plasmalogens of PE, which may account for up to 50% of mitochondrial PE, was determined. Our findings can be summarized as follows. (1) Ca2+ uptake into CL-containing liposomes was increased dramatically by PE. (2) Ca2+ entry into PC/CL liposomes was biphasic; in the presence of PE, uptake was dominated by a slow process. (3) Ca2+ uptake by PC/CL liposomes saturated at less than or equal to 2 mM external Ca2+, whereas uptake into PC/PE/CL liposomes increased with increasing Ca2+ concentration up to 10 mM or until Ca2+ release ensued. (4) Ca2+ translocation by PE-containing liposomes and the slow phase of Ca2+ uptake into PC/CL liposomes were similarly and highly dependent on temperature. It can therefore be proposed that PE amplifies the slow component of CL-mediated Ca2+ translocation. This process is characterized by a requirement for high external Ca2+ concentrations and a large apparent activation energy. Ca2+ uptake was not significantly modified by plasmalogens of PE.
Kester
M B
MB
University-Wide Toxicology Program, University of Maryland School of Medicine, Baltimore.
Sokolove
P M
PM
D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002712:Chlorides / Q000494:pharmacology; D007425:Intracellular Membranes / Q000378:metabolism*; D008081:Liposomes / Q000378:metabolism*; D008928:Mitochondria / Q000378:metabolism; D008954:Models, Biological; D010714:Phosphatidylethanolamines / Q000378:metabolism; D010955:Plasmalogens / Q000378:metabolism; D013696:Temperature
D002712:Chlorides; D008081:Liposomes; D010714:Phosphatidylethanolamines; D010955:Plasmalogens; D002118:Calcium