Purification and characterization of macromolecular phagocytosis activators released from platelets

Substantial amounts of macromolecular activators of phagocytosis from platelets (MAPPs) were released in response to exposure of platelets to the specific agonists thrombin and collagen and to calcium ionophore A23187. Centrifugation of the platelets in culture medium also resulted in a release of MAPPs, but not when the platelets were frozen and thawed prior to centrifugation. In an experiment using outdated platelet concentrates, larger and smaller MAPPs (1-MAPP and s-MAPP, respectively) were purified from the thrombin stimulated supernatant (PRPr-plasma) by sequential steps of ammonium sulfate precipitation, delipidation with ethylacetate, ConA-Sepharose affinity chromatography, MONO Q anion exchange chromatography, and Superose 12 gel filtration. This procedure yielded 59,500-fold and 63,600-fold purified 1-MAPP (0.95 mg) and s-MAPP (0.41 mg), respectively, from 1,320 ml PRPr-plasma containing 84,500 mg protein. By gradient polyacrylamide gel electrophoresis the respective molecular weights (MW) of 1-MAPP and s-MAPP were 290-320 kd and 140-160 kd; isoelectric points were 5.6 and 4.6. Both MAPPs activated neutrophil phagocytosis of IgG-SRBC at concentrations in the range of 1 pM-100 nM. Indirect enzyme-linked immunosorbent assay (ELISA) and comparisons of the concentrations required for phagocytosis activation suggested that the MAPPs were not derived from fibronectin.