A receptor binding and digestive activity of human polymorphonuclear leukocytes (PMNs) toward formyl-methionyl-leucyl-[3H]phenylalanine (3H- FMLP) was examined with the following results: Up- and down-regulation and recovery of 3H-FMLP binding activity were demonstrated. Both intact PMN and a lysate prepared from them cleaved the carboxyl terminal amino acid (phenylalanine) of 3H-FMLP. The digestive activity decreased as the receptor binding was inhibited by n-ethylmaleimide and 4- chloromercuribenzoate. Little digestive activity was found in the supernatant from PMN stimulated by FMLP. The released phenylalanine was found in the pellet and supernatant of PMNs. Digestive activity with cathepsin A-like characteristics was found in the lysate of PMN. These observations suggest that FMLP is internalized in lysosomes in a receptor-mediated manner and cleaved by the cathepsin A-like enzyme, the free phenylalanine is released extracellularly, and a part of the dissociated receptors with FMLP may return to the surface or to an intracellular receptor pool. Another finding was that the digestive activity of the lysate of cord blood granulocytes was decreased compared with that of adult blood granulocytes. This decrease may explain in part the impaired chemotaxis of cord blood granulocytes.

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