Involvement of cysteine, serotonin and their analogues in peroxidase-oxidase reactions

Myeloperoxidase in cardiovascular disease

Myeloperoxidase (MPO) plays a central role in the innate immune system by generating leukocyte-derived oxidants to combat invading pathogens. These reactive intermediates have been increasingly recognized to be potentially deleterious, causing oxidative injury in inflammatory disease states such as cardiovascular disease. Recent evidence now suggests that circulating MPO can act as a clinical prognostic indicator for patients with cardiovascular disease.

Oxidative risk for atherothrombotic cardiovascular disease

In the vasculature, reactive oxidant species, including reactive oxygen, nitrogen, or halogenating species, and thiyl, tyrosyl, or protein radicals may oxidatively modify lipids and proteins with deleterious consequences for vascular function. These biologically active free radical and nonradical species may be produced by increased activation of oxidant-generating sources and/or decreased cellular antioxidant capacity. Once formed, these species may engage in reactions to yield more potent oxidants that promote transition of the homeostatic vascular phenotype to a pathobiological state that is permissive for atherothrombogenesis. This dysfunctional vasculature is characterized by lipid peroxidation and aberrant lipid deposition, inflammation, immune cell activation, platelet activation, thrombus formation, and disturbed hemodynamic flow. Each of these pathobiological states is associated with an increase in the vascular burden of free radical species-derived oxidation products and, thereby, implicates increased oxidant stress in the pathogenesis of atherothrombotic vascular disease.

Serotonin as a physiological substrate for myeloperoxidase and its superoxide-dependent oxidation to cytotoxic tryptamine-4,5-dione

During inflammatory events, neutrophils and platelets interact to release a variety of mediators. Neutrophils generate superoxide and hydrogen peroxide, and also discharge the haem enzyme myeloperoxidase. Among numerous other mediators, platelets liberate serotonin (5-hydroxytryptamine), which is a classical neurotransmitter and vasoactive amine that has significant effects on inflammation and immunity. In the present study, we show that serotonin is a favoured substrate for myeloperoxidase because other physiological substrates for this enzyme, including chloride, did not affect its rate of oxidation. At low micromolar concentrations, serotonin enhanced hypochlorous acid production by both purified myeloperoxidase and neutrophils. At higher concentrations, it almost completely blocked the formation of hypochlorous acid. Serotonin was oxidized to a dimer by myeloperoxidase and hydrogen peroxide. It was also converted into tryptamine-4,5-dione, especially in the presence of superoxide. This toxic quinone was produced by stimulated neutrophils in a reaction that required myeloperoxidase. In plasma, stimulated human neutrophils oxidized serotonin to its dimer using the NADPH oxidase and myeloperoxidase. We propose that myeloperoxidase will oxidize serotonin at sites of inflammation. In doing so, it will impair its physiological functions and generate a toxic metabolite that will exacerbate inflammatory tissue damage. Consequently, oxidation of serotonin by myeloperoxidase may profoundly influence inflammatory processes.

Oxidative stress in phagocytes--"the enemy within"

Phagocytes represent a powerful defense system against invading microorganisms that threaten the life or functional integrity of the host. The capacity to generate and release substantial amounts of reactive oxygen species is a unique property of activated polymorphonuclear and mononuclear phagocytes. The crucial role of these molecules in killing microorganisms and their consecutive contribution to tissue damage during injury and inflammation is widely known. Although much research has been done to explore the molecular events involved in the interaction of oxygen intermediates with microbes or host tissue, surprisingly little attention has been paid to the effect of reactive metabolites on the phagocyte itself. This fact is especially surprising, since it is apparent that the activated phagocyte is directly exposed to its own toxic metabolites. The potential damage occurring during excessive radical formation might notably alter the vital functions of these primarily immunocompetent cells. Moreover, the critical role of oxygen radicals in apoptosis of leukocytes has been recently revealed. Apoptosis is now supposed to represent a key mechanism in neutrophil deactivation and resolution of inflammation. Therefore, this review will focus on the delicate balance between released oxidants and antioxidative protection within the phagocytes themselves. General and phagocyte-specific antioxidative mechanisms, which have co-evolved with the radical generating machinery of phagocytes, are discussed, since the outcome of local inflammation can directly depend on this antioxidative capacity and might range from adequate elimination of the pathogen with minimal acute tissue damage to progression towards a systemic inflammatory response syndrome.

Paracetamol (acetaminophen)-induced toxicity: molecular and biochemical mechanisms, analogues and protective approaches

An overview is presented on the molecular aspects of toxicity due to paracetamol (acetaminophen) and structural analogues. The emphasis is on four main topics, that is, bioactivation, detoxication, chemoprevention, and chemoprotection. In addition, some pharmacological and clinical aspects are discussed briefly. A general introduction is presented on the biokinetics, biotransformation, and structural modification of paracetamol. Phase II biotransformation in relation to marked species differences and interorgan transport of metabolites are described in detail, as are bioactivation by cytochrome P450 and peroxidases, two important phase I enzyme families. Hepatotoxicity is described in depth, as it is the most frequent clinical observation after paracetamol-intoxication. In this context, covalent protein binding and oxidative stress are two important initial (Stage I) events highlighted. In addition, the more recently reported nuclear effects are discussed as well as secondary events (Stage II) that spread over the whole liver and may be relevant targets for clinical treatment. The second most frequent clinical observation, renal toxicity, is described with respect to the involvement of prostaglandin synthase, N-deacetylase, cytochrome P450 and glutathione S-transferase. Lastly, mechanism-based developments of chemoprotective agents and progress in the development of structural analogues with an improved therapeutic index are outlined.

Myeloperoxidase-generated oxidants and atherosclerosis

Atherosclerosis is a chronic inflammatory process where oxidative damage within the artery wall is implicated in the pathogenesis of the disease. Mononuclear phagocytes, an inflammatory cell capable of generating a variety of oxidizing species, are early components of arterial lesions. Their normal functions include host defense and surveillance through regulated generation of diffusible radical species, reactive oxygen or nitrogen species, and HOCl (hypochlorous acid). However, under certain circumstances an excess of these oxidizing species can overwhelm local antioxidant defenses and lead to oxidant stress and oxidative tissue injury, processes implicated in the pathogenesis of atherosclerosis. This review focuses on oxidation reactions catalyzed by myeloperoxidase (MPO), an abundant heme protein secreted from activated phagocytes which is present in human atherosclerotic lesions. Over the past several years, significant evidence has accrued demonstrating that MPO is one pathway for protein and lipoprotein oxidation during the evolution of cardiovascular disease. Multiple distinct products of MPO are enriched in human atherosclerotic lesions and LDL recovered from human atheroma. However, the biological consequences of these MPO-catalyzed reactions in vivo are still unclear. Here we discuss evidence for the occurrence of MPO-catalyzed oxidation reactions in vivo and the potential role MPO plays in both normal host defenses and inflammatory diseases like atherosclerosis.

Kinetics of oxidation of serotonin by myeloperoxidase compounds I and II

The oxidation of serotonin (5-hydroxytryptamine) by the myeloperoxidase intermediates compounds I and II was investigated by using transient-state spectral and kinetic measurements at 25.0 ± 0.1°C. Rapid scan spectra demonstrated that both compound I and compound II oxidize serotonin via one-electron processes. Rate constants for these reactions were determined using both sequential-mixing and single-mixing stopped-flow techniques. The second order rate constant obtained for the one-electron reduction of compound I to compound II by serotonin is (1.7 ± 0.1) × 107 M-1·s-1, and that for compound II reduction to native enzyme is (1.4 ± 0.1) × 106 M-1·s-1 at pH 7.0. The maximum pH of the compound I reaction with serotonin occurs in the pH range 7.0-7.5. At neutral pH, the rate constant for myeloperoxidase compound I reacting with serotonin is an order of magnitude larger than for its reaction with chloride, (2.2 ± 0.2) × 106 M-1·s-1. A direct competition of serotonin with chloride for myeloperoxidase compound I oxidation was observed. Our results suggest that serotonin may have a role to protect lipoproteins from oxidation and to prevent enzymes from inactivation caused by the potent oxidants HOCl and active oxygen species.Key words: serotonin oxidation, myeloperoxidase, chloride, competition of serotonin, blood platelets, neutrophils.

The significance of the metabolism of the neurohormone melatonin: antioxidative protection and formation of bioactive substances

Recent findings suggest that the ability of melatonin to enter all body tissues and to be metabolized, enzymatically or nonenzymatically, in any of them results in a spectrum of effects, which exceed substantially those transduced by membrane receptors. These actions comprise the formation of various bioactive compounds such as N-acetylserotonin, 5-methoxytryptamine, N,N-dimethyl-5-methoxytryptamine, 5-methoxytryptophol, cyclic 2-hydroxymelatonin, pinoline, and 5-methoxylated kynuramines. Apart from enzymatic metabolism, nonenzymatic reactions with free radicals, in particular the superoxide anion and the hydroxyl radical, represent a new and significant aspect of melatonin's biological role. Melatonin represents the most potent physiological scavenger of hydroxyl radicals found to date, and recent findings suggest an essential role of this indoleamine for protection from hydroxyl radical-induced carcinogenesis and neurodegeneration.

Melatonin, hydroxyl radical-mediated oxidative damage, and aging: a hypothesis

Melatonin is a very potent and efficient endogenous radical scavenger. The pineal indolamine reacts with the highly toxic hydroxyl radical and provides on-site protection against oxidative damage to biomolecules within every cellular compartment. Melatonin acts as a primary non-enzymatic antioxidative defense against the devastating actions of the extremely reactive hydroxyl radical. Melatonin and structurally related tryptophan metabolites are evolutionary conservative molecules principally involved in the prevention of oxidative stress in organisms as different as algae and rats. The rate of aging and the time of onset of age-related diseases in rodents can be retarded by the administration of melatonin or treatments that preserve the endogenous rhythm of melatonin formation. The release of excitatory amino acids such as glutamate enhances endogenous hydroxyl radical formation. The activation of central excitatory amino acid receptors suppress melatonin synthesis and is therefore accompanied by a reduced detoxification rate of hydroxyl radicals. Aged animals and humans are melatonin-deficient and more sensitive to oxidative stress. Experiments investigating the effects of endogenous excitatory amino acid antagonists and stimulants of melatonin biosynthesis such as magnesium may finally lead to novel therapeutic approaches for the prevention of degeneration and dysdifferentiation associated with diseases related to premature aging.

Interactions of salsolinol with oxidative enzymes

Tyrosinase (EC 1.14.18.1)/O2, ceruloplasmin (human type X)/O2, and peroxidase (EC 1.11.1.7)/H2O2 oxidized the endogenous central nervous system alkaloid salsolinol (SAL) at physiological pH. The proximate oxidation product was an electrophilic ortho-quinone (4) which at pH 7.0 rapidly tautomerized. Four major initial products were formed from 4: cis- and trans-1,2,3,4-tetrahydro-1-methyl-4,6,7-isoquinolinetriol (A and B, respectively), 2,3,4-trihydro-1-methyl-7-hydroxy-6-oxyisoquinoline (C), and 1-methyl-6,7-isoquinoline diol (D). Mechanisms describing the formation of these products have been presented. Ortho-quinone 4, formed in the enzyme-mediated reactions, was rapidly attacked by glutathione to yield the 5-S-, 8-S-, and 5,8-bi-S-glutathionyl conjugates of SAL. Preliminary experiments indicated that injection of A, B and C into the CNS of mice evoked profound behavioral effects. Quinone methide C was toxic. The potential role of the oxidation of salsolinol in the neurodegenerative and behavioral effects associated with chronic alcoholism is discussed.

Oxidation of the indole nucleus of 5-hydroxytryptamine and formation of dimers in the presence of peroxidase and H2O2

5-Hydroxytryptamine (5-HT) is rapidly oxidized in the presence of peroxidase and H2O2. The major reaction product was isolated by gel chromatography and analyzed by mass spectroscopy. It is a 5-HT dimer formed under abstraction of two protons, most likely by the reaction at the C(4) position of two phenoxyradicals of 5-HT. In the presence of 5-HT an increased H2O2-consumption and a dose dependent reduction of the formation of reactive oxygen metabolites during H2O2 degradation was observed. The pattern of 5-HT reaction products separated by TLC was dependent on the H2O2 concentrations used. In the presence of albumen, plasma or tissue homogenates, massive binding of the 5-HT oxidation products to proteins was observed.