Lactoperoxidase, a dithionite ion dismutase

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Abstract
The dithionite ion is catalytically disproportionated by [cow''s milk] lactoperoxidase with Km = 0.36 mM in 100 mM glycine HCl pH 3.0. The products formed are thiosulfate and hydrogensulfite ions. The rate of reaction is considerably increased at low pH with a pKa at 3-3.5 possibly indicating the involvement of a carboxyl group. The reaction is competitively inhibited by hydrogensulfite, Ki = 5.5 mM in 100 mM glycine HCl pH 3.50. Four different spectral forms of reduced lactoperoxidase appear during the reaction. The first 2 forms are found during the lag phase of the reaction. The third form, which is interpreted as a ternary complex, exists under the dismutation phase. After exhaustion of the substrate a visible spectrum similar to that of lactoperoxidase H2O2 compound III appears. A mechanistic model for the lactoperoxidase dismutation of the dithionite ion is proposed and discussed.

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