Study of the endogenous lectins of the human platelet plasma membrane

Plasma membranes isolated from human blood platelets and human intact platelets, either fresh or previously fixed, were investigated for their possible content in intrinsic sugar-binding proteins (lectins) which can specifically bind monosaccharides such as fucose, mannose, and galactose. Light and heavy fractions of platelet plasma membrane and intact platelets fixed with 2% formaldehyde and suspended in Tyrode's buffer pH 7.0 were incubated with either L-(1-3H)-fucose, D-(1-3H)-mannose or D-(1-3H)-galactose. Intact fresh platelets were incubated either with L-(1-3H)-fucose, D-(1-3H)-mannose, D-(1-3H)-galactose, D-(1-3H)-glucose and N-acetyl-D-(1-3H)-glucosamine or with the same monosaccharides radiolabeled with (14C). Samples of radiolabeled plasma membranes and intact platelets were electrophoresed on polyacrylamide gels, and prepared for spectrometry. Results indicate that isolated human platelet plasma membranes contain sugar-binding proteins, especially for L-fucose, for which the binding values in the light fraction are about 4 times greater than for D-galactose. For the heavy fraction, the binding value for L-fucose is about 12 and 89 times higher than for D-mannose and D-galactose, respectively. Similar to plasma membrane fractions, fixed intact human platelets bind predominantly L-fucose, followed in decreasing binding values by D-mannose and D-galactose. Unlike these, fresh human platelets bind mainly D-galactose, followed in affinity values by D-glucose, L-fucose and D-mannose and to a lesser extent by N-acetyl-D-glucosamine. The binding of radiolabeled sugars to isolated platelet plasma membranes and to the previously fixed intact platelets suggests that, an endogenous lectin-like activity is expressed by the platelet membrane.