Conversion of low-affinity platelet factor 4 to β-thromboglobulin by plasmin and trypsin

Low-affinity platelet factor 4 and β-thromboglobulin are platelet-secreted proteins that bind with low affinity to heparin. They show extensive immunological cross-reactivity and appear to differ in amino acid sequence only by an amino-terminal peptide unique to low-affinity platelet factor 4. The possibility that β-thromboglobulin is derived from low-affinity platelet factor 4 by proteolysis was investigated by exposing this protein to the action of plasmin, thrombin and trypsin. While thrombin had no effect, plasmin and trypsin converted low-affinity platelet factor 4 to a species with the same electrophoretic mobility and isoelectric point as β-thromboglobulin. We conclude that β-thromboglobulin is a breakdown product of low-affinity platelet factor 4.