Preliminary Characterization of Calcium Binding in Islet-cell Plasma Membranes

Preliminary characterization of calcium binding was determined in a highly-enriched islet-cell plasma membrane fraction using a membrane filtration technique. Equilibrium calcium binding was specific, concentration dependent and saturable. Scatchard analysis indicated the existence of more than one class of calcium binding sites. The affinity constants and maximum binding capacities were 1.14 ×105 M-1 and 1.2 picomol/μg protein and 1.17× 103 M-1 and 64.8 picomol/μg for the high and low affinity sites, respectively. Bound 45Ca2+ was dissociated from the plasma membranes in a biphasic manner in the presence of excess unlabelled calcium.