Item – Theses Canada

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OCLC number
29912732
Author
Huang, Jin Tony,1959-
Title
Horseradish peroxidase catalyzed oxidation of aromatic amines and myeloperoxidase reactions with hydrogen peroxide.
Degree
Ph. D. -- University of Alberta, 1992
Publisher
Ottawa : National Library of Canada = Bibliothèque nationale du Canada, 1992.
Description
2 microfiches.
Notes
University Microfilms order no. UMI00360883.
Includes bibliographical references.
Abstract
Part I. The horseradish peroxidase (HRP) catalyzed oxidation of aromatic amines was investigated in order to obtain information of potential relevance to the in vivo activation of carcinogenic arylamines. Firstly, the kinetics of oxidation of a series of monosubstituted anilines by HRP compound II was studied. It was found that the oxidation of these anilines can be correlated by the Hammett $\sigma\rho$ equation with the value of $\rho$, the susceptibility factor, being - 6.0 $\pm$ 0.7. This result, combined with the result of a pH dependence study of these reactions, suggests that the first step of the oxidation is the removal of a hydrogen atom from the amino group. However, the rates of oxidation of the model compound for carcinogenic arylamines, 2-aminofluorene (2-AF) by HRP compounds I and II, (1.1 $\pm$ 0.5) $\times$ 10$\sp9$ M$\sp{-1}$ s$\sp{-1}$ and (1.7 $\pm$ 0.2) $\times$ 10$\sp8$ M$\sp{-1}$ s$\sp{-1}$, are about two orders of magnitudes higher than those expected on the basis of the aforementioned Hammett $\sigma\rho$ relationship, implying a different mechanism. It is therefore proposed that a cation radical is formed from 2-AF in the first step of the reaction which is stabilized by the extensive $\pi$-conjugation system of 2-AF. These results are of significance in that they provide a potential explanation for the high degree of carcinogenesis found in the polycyclic aromatic amines compared to monocyclic aromatic amines. Part II. A new mechanism for the reactions of myeloperoxidase (MPO) with hydrogen peroxide is proposed based on both qualitative and quantitative studies of these reactions. According to this mechanism, MPO compound I is formed from native MPO and hydrogen peroxide in a reversible reaction. MPO compound I further reacts with hydrogen peroxide to form compound II and superoxide. Rate constants obtained from stopped-flow experiments are (2.8 $\pm$ 0.5) $\times$ 10$\sp7$ M$\sp{-1}$ s$\sp{-1}$ for the formation of compound I and (20 $\pm$ 4) s$\sp{-1}$ for the reverse reaction, namely the decomposition of compound I to native MPO and hydrogen peroxide, and (1.6 $\pm$ 0.1) $\times$ 10$\sp5$ M$\sp{-1}$ s$\sp{-1}$ for the reaction of compound I with hydrogen peroxide. Superoxide, once formed from the reduction of compound I by hydrogen peroxide, also reduces compound I to compound II through an intermediate complex. Relevant constants were obtained through a combination of calculations based on kinetic measurements and optimizing simulations. They are 2.5 $\times$ 10$\sp9$ M$\sp{-1}$ s$\sp{-1}$ and 100 s$\sp{-1}$ for the formation and the dissociation of the compound I - superoxide complex, and 0.1 s$\sp{-1}$ for the conversion of the intermediate complex to compound II and molecular oxygen. The overall kinetics predicted in terms of the postulated mechanism and the rate constants obtained by computer simulation is consistent with the experimentally observed behavior of MPO in the presence of hydrogen peroxide.
ISBN
0315732245
9780315732247