Identification and separation of secreted platelet proteins by isoelectric focusing. Evidence that low-affinity platelet factor 4 is converted to beta-thromboglobulin by limited proteolysis

Low-affinity platelet factor 4 and beta-thromboglobulin are low molecular weight platelet secretory proteins that have common antigenic determinants. Four amino acids (Asn-Leu-Ala-Lys) at the amino terminus of beta-thromboglobulin are deleted, but the remaining sequences of the two peptides appear to be identical. Low-affinity platelet factor 4 and beta-thromboglobulin have respective isoelectric points at pH 8.0 and at pH 7.0. Identification, quantitation, and separation of both proteins was achieved by a method combining preparative isoelectric focusing and specific radioimmunoassay with anti-low-affinity platelet factor 4 antibody. It has been determined that the supernate processes immediately after platelet aggregation induced by ionophore A23187 or thrombin contains approximately 80% low-affinity platelet factor 4, 8% beta-thromboglobulin, and 12% highly cationic immunoreactive material (platelet basic protein). Experimental evidence suggests that low- affinity platelet factor 4 is originally secreted by platelets and then converted to beta-thromboglobulin by a platelet-derived, heat-labile protease that is inhibited by phenylmethylsulfonyl fluoride.