Simultaneous isolation of two platelet membrane fractions: biochemical, immunological and functional characterization

Simultaneous isolation of two platelet membrane subfractions was achieved by centrifugation on 40% sucrose from a 100.000 g crude membrane fraction. Characterization of both types of membranes was carried out by different biochemical and immunological markers. Using a surface label, 3H Concanavalin A (3HCon A), a marker enzyme, phosphodiesterase, and lipid analysis, one of the fraction has been identified as external or plasma membranes, the other consists of intracellular membranes. Further two specific antibodies directed against external membrane antigens (LeKa and IgG L) react almost exclusively with the external membranes. Finally both kinds of membranes were able to uptake calcium but the affinity for this cation was higher for the internal than for the external membranes. This suggests that both membranes are implicated in the regulation of the cytoplasmic calcium concentration and that the internal membranes (dense tubular system) play the major part in this regulation.