| The acute inflammatory response is an essential component of the mammalian host defense system that has been exquisitely evolved to defend against invading pathogens and to aid in the repair of injury. However, inflammation can also represent a pathway to promote injury and disease states, depending upon the conditions and context. It is now well-appreciated that acute inflammation modulates the expression of nearly all the genes associated with xenobiotic and endobiotic metabolism, and this has serious implications for the actions of drugs and toxicants. In general, most of the genes encoding phase 1, 2 and 3 metabolic enzymes are down-regulated during acute inflammation. Whereas intense focus has been placed on these metabolic enzyme systems, the role of neutrophils in the metabolic abnormalities associated with acute inflammation has received limited attention. Neutrophils are the most abundant leukocytes in mammalian organisms, and they are equipped with a number of oxidative and proteolytic pathways that could contribute to altered metabolism during acute inflammation. Myeloperoxidase (MPO), a highly abundant hemoprotein/enzyme expressed by neutrophils, is one such system that has been implicated in the oxidative metabolism of various drugs. In fact, MPO has been implicated as a causative player in many idiosyncratic drug reactions and the ensuing side effects. While most studies have revealed that biotransformation of drugs by MPO to a 'reactive' intermediate that binds to proteins has been established, the precise mechanisms and biological implications have remained poorly characterized. Herein, I provide experimental evidence demonstrating that MPO can oxidatively modify p-cresol, an exemplary phenolic xenobiotic/endobiotic compound, to a species that possesses bioactivity relevant to inflammatory states not shared with the parent compound. In Chapter 1 of this dissertation, I review the basic concepts of inflammation, the role of inflammation in modulating the expression of the cellular machinery responsible for metabolism of xenobiotics and endobiotics, and summarize the current literature revealing a potential role for MPO in metabolizing drugs and xenobiotics. In Chapter 2, evidence is provided which demonstrate novel mechanisms for MPO-mediated oxidative and nitrative modification of p-cresol; most notably ring nitrated p-cresol (2-nitro-4-methylphenol), di-cresol, and Pummerer's ketone. Pummerer's ketone was found to be the predominant metabolite formed by MPO, and this species displayed the capacity to activate the aryl hydrocarbon receptor (AhR), albeit as a weak ligand. Data is also presented which disputes the production of free hypochlorous acid (HOCl) by MPO-mediated oxidation of chloride (Cl-). The data, in fact, reveal that MPO chlorinates small organic phenolic compounds by an enzyme-bound chlorinating intermediate, which has significant implications for further understanding the functions of MPO in vivo. In Chapter 3, I demonstrate that Pummerer's ketone serves as a relatively potent activator of the transcription factor, Nrf2, in both murinc and human cell lines, and this leads to potent transcriptional and translational expression of the heme oxygenase-1 gene. Chemical and mass spectrometric studies revealed that the alpha,beta-unsaturated ketone moiety contained within the structure of Pummerer's ketone may be crucial for its' biological actions by forming adducts with protein and non-protein sullhydryl groups via Michael addition reactions. Collectively, the data presented herein reveal that MPO can play a contributing role in the oxidative metabolism of p-cresol, and other phenolic compounds, and that this may contribute to the altered metabolism observed during inflammation. |
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