Bromoperoxidase specific activity

Bromoperoxidase activity is expressed as micromoles of mcd brominated per minute per milligram of enzyme (U/mg). The early work on V-BrPO employed the oxidation of iodide by dihydrogen peroxide [1], forming triiodide (Ir), which was followed spectrophotometrically at 353 nm ( = 26,400 M 1cm 1). However, this reaction is less desirable for quantitation of haloperoxidase activity because of competing side reactions, such as the nonenzymatic oxidation of iodide by dihydrogen peroxide and reduction of triiodide by dihydrogen peroxide (discussed later). The specific activity of mcd bromination for V-BrPO isolated from A. nodosum is 170 U/mg (at pH 6.5, 2 mM H202, 0.1 M Br , 50 nM mcd,... 

The specific chloroperoxidase, bromoperoxidase, and iodoperoxi-dase activities differ substantially and depend on pH and the concentrations of halide and hydrogen peroxide (2). In general, the specific activity for halide oxidation increases in the order of chloride, bromide, and iodide. The pH for maximum specific haloperoxidase activity generally decreases in the order of iodide, bromide, and chloride, but direct comparisons are difficult because the pH maximum can be shifted over several pH units by varying the ratio of halide to hydrogen peroxide and because both halide and hydrogen peroxide can inhibit the enzyme under certain conditions. 

In addition to bromide and iodide, V-BrPO can catalyze the oxidation of chloride [64]. As mentioned previously and discussed more fully later, a distinct enzyme, vanadium chloroperoxidase, has also been discovered. Originally it was thought that V-BrPO could only catalyze the oxidation of bromide and iodide by dihydrogen peroxide. In fact, under the standard mcd bromoperoxidase assay conditions, in which the V-BrPO concentration is ca. nanomolar, very little, if any, chlorination of mcd is observed. However, it seemed very unusual that V-BrPO could be inhibited by fluoride and bromide, but apparently not by chloride [27], In reinvestigating the halide specificity of V-BrPO, it was discovered that when the enzyme concentration is increased 100-fold to 0.1 pM, chlorination is observed at an appreciable rate [64], The specific chloroperoxidase activity is 0.76 U/mg (under conditions of 1 M certified 100% bromide-free KC1, 2 mMH202, 50 pM... 

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