Temperature dependence of [formula omitted] exchange activity in beef-heart sarcolemmal vesicles and proteoliposomes

Abstract

Temperature dependence of Na + Ca 2+ exchange activity was studied in beef cardiac sarcolemmal vesicles in the absence and presence of the inhibitor amiloride and in proteoliposomes reconstituted with different lipid mixtures. Arrhenius plots for Na + Ca 2+ exchange activity in both control and amiloride-treated vesicles revealed an apparent energy of activation of 9665 ± 585 (SE, n = 4) cal/mol, corresponding to a temperature coefficient ( Q 10) value of 1.70 ± 0.05 (SE, n = 4) over the range 25–37 °C. When Na + Ca 2+ exchange was reconstituted into phosphatidylcholine (PC):phosphatidylserine (PS) (52:48, mol/mol), PC:PS:cholesterol (25: 39:36, mol/mol), and PC:PS:distearoylphosphatidylcholine (DSPC) (31:48:21, mol/mol) proteoliposomes, the highest activity was found in PC:PS:cholesterol proteoliposomes. Arrhenius plots of Na + Ca 2+ exchange activity exhibited breakpoints at 23 °C (PC:PS), 33 °C (PC:PS:cholesterol), and 23 °C (PC:PS:DSPC). The increase in the thermotropic transition temperature with cholesterol could result from the condensing effect of this sterol, whereas the breaks observed with PC:PS and PC:PS:DSPC could be caused by a non-lipid-mediated membrane protein conformational change. These results indicate that the lipid microenvironment around the Na + Ca 2+ exchanger and the nature of the specific lipid-protein interactions influence the activity of this antiporter. Further evidence supporting the hypothesis that cholesterol behaves as a specific positive effector for the exchanger is also given.