Cofactor Role of Iodide in Peroxidase Antimicrobial Action Against Escherichia coli

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Abstract
The mechanism of antimicrobial activity of the peroxidase-hydrogen peroxide (H 2 O 2 )-iodide (I − ) system was investigated. Inhibition of respiration and loss of viability of Escherichia coli were used as measures of antimicrobial activity. Because the bacteria destroyed H 2 O 2 , peroxidase antimicrobial action depended on the competition for H 2 O 2 between the bacteria and the peroxidase. Utilization of H 2 O 2 by the peroxidase was favored by (i) increasing either the peroxidase or the I − concentration, so as to increase the rate of oxidation of I − , (ii) lowering the temperature to lower the rate of destruction of H 2 O 2 by the bacteria, and (iii) adding H 2 O 2 in small increments so as to avoid a large excess of H 2 O 2 relative to I − . When utilization of H 2 O 2 by the peroxidase system was favored, the peroxidase system and iodine (I 2 ) were equivalent. That is, antimicrobial action per mole of H 2 O 2 equaled that per mole of I 2 . Also, identical antimicrobial action was obtained either by incubating the bacteria directly with the peroxidase system or by preincubating the peroxidase system so as to form I 2 and then adding the bacteria. On the other hand, peroxidase antimicrobial action could be obtained at low I − concentrations. These I − concentrations were lower than the concentration of I 2 that was required for antimicrobial action. It is proposed that peroxidase-catalyzed oxidation of I − yields I 2 , which reacts with bacterial components to yield the oxidized components and I − . The I − that is released can be reoxidized and participate again in the oxidation of bacterial components. In this way, I − acts as a cofactor in the peroxidase-catalyzed oxidation of bacterial components.