Role of the Phagocyte in Host-Parasite Interactions XXVII. Myeloperoxidase-H 2 O 2 -Cl − -Mediated Aldehyde Formation and Its Relationship to Antimicrobial Activity

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Abstract
Evidence is presented which suggests that the mechanism of action of the myeloperoxidase-H 2 O 2 -Cl − antimicrobial system in the phagocyte is by the formation of aldehydes. Aldehyde production resulting from myeloperoxidase-mediated decarboxylation and deamination of alanine was quantitated with 20,000-g granules from guinea pig polymorphonuclear leukocytes serving as the enzyme. Equimolar quantities of acetaldehyde and CO 2 were obtained. There was an absolute requirement for both H 2 O 2 and Cl − for decarboxylation by the myeloperoxidase-containing granules. The myeloperoxidase-H 2 O 2 -Cl − system decarboxylated both d - or l -alanine equally and had a p H optimum of 5.3. Decarboxylation of l -alanine by intact guinea pig polymorphonuclear leukocytes was increased 2.5-fold by phagocytosis. Guaiacol peroxidation by the granules was inhibited 90% in the presence of Cl − at acid p H. Under these conditions, decarboxylation and deamination of amino acids by myeloperoxidase were significantly stimulated, resulting in aldehyde production. Taurine, a competitive inhibitor of amino acid decarboxylation, inhibited bactericidal activity of the myeloperoxidase-H 2 O 2 -Cl − system but had no effect on the myeloperoxidase-H 2 O 2 -I − bactericidal system. Since the myeloperoxidase-H 2 O 2 -I − system does not participate in amino acid decarboxylation, its mechanism of antimicrobial action appears to be different from that found with Cl − .

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