Biological reactivity of hypochlorous acid: implications for microbicidal mechanisms of leukocyte myeloperoxidase.

Abstract

Oxidative degradation of biological substrates by hypochlorous acid has been examined under reaction conditions similar to those found in active phagosomes. Iron sulfur proteins are bleached extremely rapidly, followed in decreasing order by f3carotene, nucleotides, porphyrins, and heme proteins. Enzymes containing essential cysteine molecules are inactivated with an effectiveness that roughly parallels the nucleophilic reactivities of their sulfhydryl groups. Other compounds, including glucosamines, qumiones, riboflavin, and, except for N-chlorination, phospholipids, are unreactive. Rapid irreversible oxidation of cytochromes, adenine nucleotides, and carotene pigments occurs when bacterial cells are exposed to exogenous hypochlorous acid; with Escherichia coli, titrimetric oxidation of cytochrome was found to coincide with loss of aerobic respiration. The occurrence of these cellular reactions implicates hypochlorous acid as a primary microbicide in myelo.roxidase-containing leukocytes; the reactivity patterns observed are consistent with the view that bactericidal action results primarily from loss of energy-linked respiration due to destruction of cellular electron transport chains and the adenine nucleotide pool.Bacteria commonly lose their ability to divide within minutes of encountering phagocytosing leukocytes (1). Loss ofcell viability often occurs well before the onset ofcellular digestion as determined by physiological changes (2), macromolecular degradation (3, 4), or loss of macromolecular biosynthesis (5). The specific reactions giving rise to cellular death have not yet been identified, at least in part because leukocytes are capable ofinitiating a diverse set of processes which are potentially lethal (6, 7). Given the above observations, however, the microbicidal reactions must be among the first that attend interaction with the leukocyte.Reactions catalyzed by myeloperoxidase (MPOase) appear to make major contributions to the microbicidal-action of polymorphonuclear leukocytes (PMNs) (6, 7). The cell-free MPOase-H202-Cl-system is 'potentially microbicidal; chlorination of bacteria by the cell-free system (8, 9) and of macromolecular fractions during PMN digestion of bacteria (9) electrophile-nucleophile interactions involving association of electropositive chlorine with electron-rich centers on the substrate; reaction pathways are-correspondingly highly dependent upon medium-conditions, particularly H' and Cl-concentrations (14). With the exception ofamines and amino acids (11,12,15), the reactions of biological compounds with HOC1 are not understood (16).We report 'here the results of a survey of simple biological compounds which can be taken as prototypic ofvarious components of bacterial cells. :The data demonstrate that HOCI is strongly selective -toward nucleotides and compounds that are models for certain components ofrespiratory redox chains. This selectivity is shownito extend to bacterial cells. Table 1 were -determined from loss of their characteristic absorption bands when ...

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