Lactoperoxidase, a dithionite ion dismutase.

The dithionite ion is catalytically disproportionated by 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 two 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.