Reduction of lactoperoxidase by the dithionite anion monomer

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Abstract
The reduction of lactoperoxidase with sodium dithionite has been studied by means of stopped-flow spectrophotometry in an anaerobic system. Under pseudo-first-order conditions, the rate constant was found to be linearly dependent on the square root of the dithionite concentration, which confirms the monomeric radical, .**GRAPHIC**. as the reducing species. The second-order rate constant is moderately influenced by increased ionic strength but drastically increased at lower pH. The pH dependence supports the previously suggested existence of a carboxyl group, essential to the different enzymatic functions of lactoperoxidase. The second-order rate constant for the reduction of lactoperoxidase at pH 7.0 (k1 = 1.3 .times. 105 M-1 s-1) was about three times higher than the rate constant for the reduction of cyanide-bound lactoperoxidase and two times the rate constant for the reduction of the fluoride-lactoperoxidase complex.