No abstract

The kinetics of the reactions of hydrogen peroxide and cyanide with native horseradish peroxidase, as me11 as reactions of compounds I and IT with ferrocyanide have been studied in ordinary water and in deuteriuni oxide at 25'C and ionic strength 0.1 1 \:sing a stopped-flow apparatus. Rate constants for all reactions were measurcd over a wide range of acidity in both solvents from which equilibriunl and kinetic isotope effccts \\ere evaluated. Protonation of an ionizable group on the enzyme with a pK., value of 4.15 r 0.05 in ~vater inhibits the reactions with both hydrogen peroxide and cyanide. A significant kinetic isotope effect, k,, k , = 1.6 i 0.1, mas measured for compound 1 formation ahcreai no significant kinetic isotcpe effect was found for cyanide binding. On the basis of these findings, a partial niechanisn~ for compound 1 formation is proposed in which the group of pK, 4.15 plays a crucial role. The pH dependencies of the ferrocyanide reaction in the pH interval 4.5-10.8 confirnled the role of an acid group with a pK, of5.2 for compound I and for compound 11 a pK, of 8.6 and another with a val~ie loner than that encolupassed hy the pH range of the study. Eq~lilibri~lm isotope effects Mere found but no kinetic isotope effects for either the reaction of compound I or of compound 11. This suggests that there are no rate-limiting proton transfers in the reactions between ferrocyanide and compounds I and 11 of horseradish peroxidase. The only redcrcing substrates which cvhihit positive I<,, k , values possess a labile proton. H. BRIAU UL\FORD, W. DO\ALD HLWSON et HXKAN S T~I \~R .Can. j. Chem. 56.2844Chem. 56. (1978 Faisaat appel B un appareil B flux stoppi., on a Ctudie la cinetique des reactions du peroxyde

Using the facilities of the RIV Alpha Helix, marine algae were collected and immediately sampled for the presence of peroxidase enzymes capable of performing electrophilic bromination via the oxidation of bromide ion. Algae were also sampled for halogenated neutral lipids. A spectrophotometric assay employing monochlorodimedone was used to measure the level of enzymatic bromination, and an automated halogen analyzer was used to determine the level of lipid halogen. Of 72 algae tested for bromoperoxidase, 55 were active; moreover 27 of 102 algae contained significant levels of lipid halogen. For 67 algae there was sufficient sample to complete both assays. A comparison of the reds, browns and greens showed the Rhodophyta to be the richest in both bromoperoxidase and lipid halogen levels while the Phaeophyta was the poorest. pH optima in the range pH 5 to 8 were determined for a few of the more active algal bromoperoxidases. Cyanide inhibited the bromination of monochlorodimedone but inhibition could not be distinguished from the possibility of competition by cyanide for electrophilic bromination.

No abstract

The rate of formation of compound I from the reaction of native horseradish peroxidase with hydrogen peroxide was studied from 3.7–70.0 °C. The second-order rate constants were used to construct an Arrhenius plot from which the activation energy of this reaction was calculated to be 3.5 ± 1.0 kcal/mol. The irreversibility of the reaction at 25 °C was confirmed by comparing absolute absorbance changes as recorded by the stopped-flow apparatus with the known spectra of the native enzyme and compound I.