A novel function of transferrin as a constituent of macromolecular activators of phagocytosis from platelets and their precursors.

Macromolecular activators of phagocytosis from platelets (MAPP:s-MAPP, 1500kDa and 1-MAPP, 300kDa) are glycoproteins released from human platelets and activate leukocyte phagocytosis via the Fc gamma receptors. Their production can be shown in CPD-stored platelets which have lost MAPP releasing activity by incubation with plasma-derived precursos of MAPP (pre-MAPP : pre-s-MAPP, 150kDa and pre-l-MAPP, 300kDa) in the presence of Ca++. Partial amino acid sequence analysis of s-MAPP revealed that it had homogeneity with transferrin (TF). Affinity chromatography using anti TF immunosorbent column showed that all of pre-MAPP and MAPP had immunoreactivity with anti TF antibodies. S-MAPP and 1-MAPP rich preparations from platelet release products lost their activity after treatment with ATP at acidic pH, in which condition iron atoms could be removed from holo-transferrin molecules. In the experiment using polymerized TF and stored platelets, it was shown that platelets incubated with dimer and tetramer TF could produce MAPP function. These results suggest that dimer and tetramer transferrin are precursors of s-MAPP and 1-MAPP, respectively and that iron atoms are necessary for their phagocytosis activating function.