Surface charge and calcium binding in sarcoplasmic reticulum membranes

Abstract

Vesicles of fragmented sarcoplasmic reticulum membranes have been adsorbed on to 2.68μ latex spheres. Observation of these vesicle containing spheres in the presence of an electric field allows a calculation of the electrophoretic mobility of the vesicles Following this determination, the net membrane surface charge has been estimated. The mobility of sarcoplasmic reticulum membranes exhibited a dependency on pH. At an ionic strength of 0.10 a mobility (pH=7.0) of −0.67±0.10μ/sec/volt/cm was observed. At pH=7.0 and Г/2=0.150 the net excess negative charge density was 2.0×10⁻² coul/m². This is equivalent to one charge per 10³ A² (assuming a uniform charge distribution). With an average vesicle volume of 2.8×10⁸ A³ and a surface area of 2×10⁶ A² the surface of one vesicle would contain a net of approximately 2×10³ negative charges. While the mobility did not change during uptake of calcium by the vesicles, both glutaraldehyde fixation and lecithin extraction by phospholipase C greatly altered the mobility of the vesicle membrane. Calcium binding and uptake both exhibited a dependence on pH.