On the Relationship Between Low Affinity PF₄ and ß-Thromboglobulin

Platelets stimulated by thrombin or Ca-ionophore A 23187 release a number of proteins which bind specifically to heparin. Those with low affinity (displacement by 0.5M NaCl) include low affinity PF₄ (LA-PF₄) and β-thromboglobulin (βTG). LA-PF₄ and βTG are closely related as shown by their comparable affinity for heparin, immunological cross-reactivity and the similarity of the limited amino acid sequences which have been compared. The latter suggest that LA-PF₄ differs from βTG only by a 4-residue NH₂-terminal sequence (asn-leu-ala-lys-) unique to LA-PF₄. Isoelectric focussing and disc gel electrophoresis, which distinguish the two proteins, were used to characterise preparations from various laboratories. That different isolation procedures yielded predominantly one or other species implies that βTG may be derived from the larger LA-PF₄ by proteolysis. Direct evidence was sought by exposing pure LA-PF₄ to thrombin and plasmin for 1 h at 37°. While thrombin (40 u/ml) had no effect, highly purified plasmin (2 u/ml) converted LA-PF₄ quantitatively to a component with mobility and isoelectric point close to βTG. The alteration in charge resulted only from the cleavage of terminal residues since the mobility on SDS-gels was unchanged. Our evidence therefore suggests that βTG is a stable proteolytic fragment of LA-PF₄. Whether this conversion occurs in vivo, and involves platelet-associated proteases, is under investigation.