Crossed Immunoelectrophoresis In The Identification Of Platelet Membrane Glycoproteins And Complexes

Abstract

Human platelet membranes were solubilized in 1% Triton X-100 and subjected to crossed immunoelectrophoresis, employing a rabbit anti-piatelet membrane antibody. Ten different antigens were observed fairly consistently; one could be identified as albumin, the other as fibrinogen. Surface antigens were determined by antibody adsorbtion experiments, and cell surface labeling with 125I-lactoperoxidase. Four surface antigens reacted with concanavalin A, when this was employed as an intermediate spacer gel. A major surface antigen, 10, was present on all preparations and was inversely related to antigens 13 and 18, which moved more cathodally. Membranes from preparations with full 10 antigen peaks had absent or diminished 13 and 18 antigen peaks, whereas preparations with absent to incomplete cathodal curves had increased 13 and 18 antigen peaks. Digestion of intact washed platelets with α chymotrypsin resulted in a decrease in the 10 antigen peak and an increase in 13 and 18, suggesting a structural relationship. Extraction of platelet membranes in EDTA or EGTA resulted in the disappearance of 10 and appearance of 13 and 18 (which react with concanavalin A) and 15 and 16 (which do not). Splitting of the major antigen into 13, 18, 15 and 16 could be prevented by addition of excess Ca++ relative to EGTA (but not by excess Mg++). Similar results were obtained in the absence of chelating agents following dialysis at 60-200 hrs against ‘Ca++-free’ buffer. Five patients with Glanzmann’s thrombasthenia have been studied. All k components of the major membrane antigen are missing. We conclude that the major antigen, 10, is composed of 2 glycoproteins, 13 and 18, and 2 other surface proteins, 15 and 16, which are held together by Ca++. It is conceivable that patients with Glanzmann’s thrombasthenia are lacking a membrane receptor for Ca++ or the platelet membrane (glyco)protein which anchors these A components.