Oxidation of amino acids by human neutrophils

We studied the oxidation of alanine and methionine by human neutrophils. Phagocytosis enhanced the decarboxylation of amino acids by human neutrophils. Decarboxylation of amino acids was dependent on the myeloper-oxidase system (MPO-H₂O₂-Cl^–). This was further confirmed using purified canine MPO. Human neutrophils and the MPO system were about 10 times more efficient in decarboxylating alanine than methionine. They also oxidized methionine to methionine sulfoxide. The fraction of methionine decarboxylated by human neutrophils or the MPO system was small compared to the fraction which was oxidized to methionine sulfoxide. Thus methionine was preferentially oxidized to methionine sulfoxide by the MPO system. However, once methionine was oxidized to methionine sulfoxide, it was readily decarboxylated by the MPO system. The results suggest that the thio group of methionine prevents its carboxylic group from being decarboxylated.