A colorimetric assay for steady-state analyses of iodo- and bromoperoxidase activities.

Abstract The standard assay for iodoperoxidase activity is based on the spectrophotometric detection of triiodide formed during the enzymatic reaction. However, chemical instability of I 3 - has limited the method to high iodide concentrations and acidic conditions. Here we describe a simple spectrophotometric assay for the determination of iodoperoxidase activities of vanadium haloperoxidases based on the halogenation of thymol blue. The relation between color and chemical entities produced by the vHPO/H 2 O 2 /I − catalytic system was characterized. The method was extended to bromine and, for the first time, allowed measurement of both iodo- and bromoperoxidase activities using the same assay. The kinetic parameters ( K m and k cat ) of bromide and iodide for vanadium bromoperoxidase from Ascophyllum nodosum were determined at pH 8.0 from steady-state kinetic analyses. The results are concordant with an ordered two-substrate mechanism. It is proposed that halide selectivity is guided by the chemical reactivity of peroxovanadium intermediate rather than substrate binding. This method is superior to the standard I 3 - assay, and we believe that it will find applications for the characterization of other vanadium as well as heme haloperoxidases.