Oxidative decarboxylation of free and peptide-linked amino acids in phagocytizing guinea pig granulocytes

Abstract

The oxidative decarboxylation of amino acids by a system consisting of myeloperoxidase-hydrogen peroxide-chloride has been demonstrated previously by others and the process has been considered to be part of the microbicidal armamentarium of some phagocytic leukocytes. We were able to translate these earlier observations, made on model systems, to intact guinea pig granulocytes. We could demonstrate differences in the cellular handling of peptide-linked amino acids as particles, compared with free amino acids. Specific inhibitors were used to explore two routes of oxidative decarboxylation: (a) the myeloperoxidase-catalyzed direct decarboxylation-deamination reaction, and (b) oxidation of alpha-keto acids after transamination of amino acids. These inhibitors were cyanide, azide, and tapazole for the former pathway, and amino-oxyacetate for the latter. Amino-oxyacetate profoundly inhibited the decarboxylation of free 14C-amino acids (alanine and aspartate) in both resting and stimulated cells, but had only a minimal effect on 14CO2 production from ingested insoluble 14C-protein. On the other hand, the peroxidase inhibitors cyanide, azide, and tapazole dramatically inhibited the production of 14CO2 from ingested particulate 14C-protein, but had only small effects on the decarboxylation of free amino acid. Soluble, uniformly labeled 14C-protein was not significantly converted to 14CO2 even in the presence of phagocytizable polystyrene beads. These observation suggest that the amino acids taken up by phagocytosis (e.g., as denatured protein particles) are oxidatively decarboxylated and deaminated in the phagosomes by the myeloperoxidase-hydrogen peroxide-chloride system; soluble free amino acids that enter the cytoplasm by diffusion or transport are oxidatively decarboxylated after transamination by the normal cellular amino acid oxidative pathway.