The mechanism of peroxidase-mediated cytotoxicity. II. Role of the heme moiety

Heme Synthesis and Acquisition in Bacterial Pathogens

Bacterial pathogens require the iron-containing cofactor heme to cause disease. Heme is essential to the function of hemoproteins, which are involved in energy generation by the electron transport chain, detoxification of host immune effectors, and other processes. During infection, bacterial pathogens must synthesize heme or acquire heme from the host; however, host heme is sequestered in high-affinity hemoproteins. Pathogens have evolved elaborate strategies to acquire heme from host sources, particularly hemoglobin, and both heme acquisition and synthesis are important for pathogenesis. Paradoxically, excess heme is toxic to bacteria and pathogens must rely on heme detoxification strategies. Heme is a key nutrient in the struggle for survival between host and pathogen, and its study has offered significant insight into the molecular mechanisms of bacterial pathogenesis.

Physical and catalytic properties of a peroxidase derived from cytochrome c

Except for its redox properties, cytochrome c is an inert protein. However, dissociation of the bond between methionine-80 and the heme iron converts the cytochrome into a peroxidase. Dissociation is accomplished by subjecting the cytochrome to various conditions, including proteolysis and hydrogen peroxide (H(2)O(2))-mediated oxidation. In affected cells of various neurological diseases, including Parkinson's disease, cytochrome c is released from the mitochondrial membrane and enters the cytosol. In the cytosol cytochrome c is exposed to cellular proteases and to H(2)O(2) produced by dysfunctional mitochondria and activated microglial cells. These could promote the formation of the peroxidase form of cytochrome c. In this study we investigated the catalytic and cytolytic properties of the peroxidase form of cytochrome c. These properties are qualitatively similar to those of other heme-containing peroxidases. Dopamine as well as sulfhydryl group-containing metabolites, including reduced glutathione and coenzyme A, are readily oxidized in the presence of H(2)O(2). This peroxidase also has cytolytic properties similar to myeloperoxidase, lactoperoxidase, and horseradish peroxidase. Cytolysis is inhibited by various reducing agents, including dopamine. Our data show that the peroxidase form of cytochrome c has catalytic and cytolytic properties that could account for at least some of the damage that leads to neuronal death in the parkinsonian brain.

Role of peroxidases in Parkinson disease: a hypothesis

Extensive research has been done to elucidate the underlying molecular events causing neurodegenerative diseases such as Parkinson disease, yet the cause and the individual steps in the progression of such diseases are still unknown. Here we advance the hypothesis that, rather than or in addition to inorganic radical molecules, heme-containing peroxidase enzymes may play a major role in the etiology of Parkinson disease. This hypothesis is based on the following considerations: (1) several heme-containing enzymes with peroxidase activity are present in the substantia nigra pars compacta; (2) these peroxidases have the ability to catalyze the oxidation of proteins and lipids; (3) certain heme peroxidases are known to destroy cells in vivo; (4) heme peroxidases have the stability and specificity that could account for the fact that specific molecules and cells are subject to damage in Parkinson disease, rather than a random destruction; (5) heme peroxidase activity could account for certain reactions in connection with parkinsonism that thus far have not been adequately explained; and (6) the participation of a heme peroxidase could explain some recent observations that are inconsistent with the oxyradical theory. The peroxidase-catalyzed oxidative pathway proposed here does not preclude the participation of apoptosis as an additional mechanism for cell destruction.

The cytotoxic activity of lactoperoxidase: enhancement and inhibition by neuroactive compounds

Neuronal death associated with Parkinson's disease is commonly believed to be caused by oxygen- and nitrogen-derived free radical species. Some years ago, however, we showed that peroxidase from the midbrain of dogs is able to kill various cell types, including neuroblastoma cells (M. B. Grisham et al., J. Neurochem. 48: 876-882: 1987). We postulated that a nigral peroxidase may play a significant role in the degeneration of dopaminergic neurons in Parkinson's disease. To further establish proof of principle, we recently performed a series of experiments using horseradish peroxidase and lactoperoxidase. We showed that the cytotoxic activity of lactoperoxidase is fully inhibited by physiological concentrations of dopamine, reduced glutathione, and L-cysteine, as well as by micromolar concentrations of apomorphine, desferal, aspirin, and uric acid. l-Methyl-4-phenyl-1,2-dihydropyridine (MPDP) and l-methyl-4-phenylpyridinium (MPP+) augment the cytotoxic activity, whereas l-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, deprenyl, and pargyline had minimal or no effect. We also showed that horseradish peroxidase catalyzes the oxidation of MPDP to MPP+. Thus, contrary to the generally accepted theory that the in vivo oxidation of MPDP occurs spontaneously, this reaction may be catalyzed by a brain peroxidase. These observations lend further support to the suggestion that a brain peroxidase may play an important role in the metabolic events associated with Parkinson's disease.

A novel hemoglobin-adenosine-glutathione based blood substitute: evaluation of its effects on human blood ex vivo

Chemically modified hemoglobin (Hb) solutions are under current investigation as potential red cell substitutes. Researchers at Texas Tech University have developed a novel free Hb based blood substitute product. This blood substitute is composed of purified bovine Hb cross-linked intramolecularly with o-adenosine-5'-triphosphate and intermolecularly with o-adenosine, and conjugated with reduced glutathione (GSH). In this study, we compared the effects of our novel blood substitute and unmodified (U) Hb, by using allogenic plasma as the control, on human blood components: red blood cells (RBCs), platelets, monocytes (Mo), and low-density lipoproteins (LDLs). The pro-oxidant potential of both Hb solutions on RBCs was examined by the measurement of osmotic and mechanical fragility, conjugated dienes (CD), lipid hydroperoxides (LOOH), thiobarbituric acid reactants (TBAR-S), isoprostanes (8-iso PGF2alpha) and intracellular GSH. The oxidative modification of LDLs was assessed by CD, LOOH, and TBAR-S, and the degree of apolipoprotein (apo) B cross-linking. The effects of Hb on platelets have been studied by monitoring their responses to the aggregation agonists: collagen, ADP, epinephrine, and arachidonic acid. Monocytes were cultured with Hb solutions or plasma and tested for TNF-alpha and IL-1beta release, then examined by electron microscopy. Results indicate that native UHb initiates oxidative stress of many blood components and aggravates inflammatory responses of Mo. It also caused an increase in RBC osmotic and mechanical fragility (p < 0.001). While the level of GSH was slightly changed, the lipid peroxidation of RBC increased (p < 0.001). UHb was found to be a stimulator of 8-iso PGF2alpha synthesis, a potent modulator of LDLs, and an effective potentiator of agonist induced platelet aggregation. Contrarily, our novel blood substitute did not seem to induce oxidative stress nor to increase Mo inflammatory reactions. The osmotic and mechanical fragility of RBCs was similar to that of the control. Such modified Hb failed to alter LDLs, increase the production of 8-iso PGF2alpha, but markedly inhibited platelet aggregation. The effect of this novel blood substitute can be linked with the cytoprotective and anti-inflammatory properties of adenosine, which is used as a cross-linker and surface modifier, and a modification procedure that lowers the hemoglobin pro-oxidant potential.

The cytotoxic activities of human hemoglobin and diaspirin crosslinked hemoglobin

It is well known that hemoglobin (Hb) possesses many oxidative enzyme activities, including a pseudo-peroxidase activity. It has also been shown by many investigators that various peroxidases in the presence of hydrogen peroxide and a halide ion exert a potent cytotoxic activity toward various mammalian cell types. It has further been observed by various investigators that the administration of relatively large amounts of purified Hb or a Hb derivative to a host animal during resuscitation experiments leads to a number of unrelated types of tissue damage and cell damage in the host. The first objective of this investigation was to determine if the observed tissue and cell damage may be due to a cytotoxic activity that Hb may exert in vivo analogous to that of the peroxidases. We also showed some time ago that peroxidases are able to activate peritoneal macrophages to the cytocidal state. Hence, we also addressed the question whether or not Hb is able to activate macrophages in a similar manner. Our results were negative with regard to both questions. Further investigations indicated that, unlike the peroxidases, ferryl-Hb is unable to oxidize iodide to iodine at a measurable rate, which appears to be the reason for the lack of cytotoxic activity.

Peroxidase deficiency of nickel-transformed hamster cells correlates with their increased resistance to cytotoxicity of peroxides

Using a procedure aimed at isolation of genes that are inactivated during nickel-induced carcinogenesis in Chinese hamster cells, a homolog of genes encoding human and mouse heme containing peroxidases has been cloned. Northern blot analysis of normal cultured fibroblasts and two nickel-transformed cell lines confirmed that this gene was expressed in normal but not in transformed cells. Nickel-transformed cells also tested negative for peroxidase activity using a sensitive fluorescence assay. Cultured embryo cells or fibroblasts that express peroxidase activity and their nickel-transformed peroxidase-deficient counterparts were employed to investigate the role of peroxidase-catalyzed processes in cytotoxicity induced by tert-butyl hydroperoxide or cumene hydroperoxide. It has been found that peroxidase-deficient cells were significantly more resistant to cytotoxic effect of these compounds suggesting that cytotoxic effect of hydroperoxides may be mediated in part by free radicals generated in the course of peroxidase-catalyzed reactions.

Bactericidal effects of photoactivated porphyrins--an alternative approach to antimicrobial drugs

Photoactivated porphyrins display a potent cytotoxic activity towards a variety of Gram positive bacteria, mycoplasma and yeasts, but not Gram negative cells. The prerequisite for photosensitization of a microbial cell is the binding of porphyrin to the cytoplasmic membrane in a pH-dependent manner. On illumination, the membrane bound, and possibly, cytoplasmic porphyrin molecules generate singlet oxygen and radicals which sensitize biomolecules and lead to cell death. The immediate inhibition of cell growth on photodynamic treatment is accompanied by alterations in cell wall and membrane synthesis, leading to the formation of large mesosomes adjacent to the unaccomplished septa. Hemin bound to microbial cells exerts cytotoxic activity by peroxidative and oxidative reactions independent of light. Future research in the field may enhance the possibility of using porphyrin photosensitization for treatment of microbial infections. Such clinical use will be unrelated to the antibiotic resistance of the pathogen. Resistance of Gram negative bacteria to porphyrin photosensitization is the main impediment to its use as a broad spectrum antibacterial method.

Vanadate as an inhibitor of plant and mammalian peroxidases

Vanadate ions are shown to inhibit horseradish, squash, and rat intestinal peroxidases by following the reaction spectrophotometrically in a wide range of vanadate concentrations. I50 in phosphate buffer were 43, 9.4, and 535 microM, respectively. No inhibitory effect was found on cow milk lactoperoxidase and beef liver catalase. Gel filtration of peroxidases in the presence of vanadate, as carried out by radioactive 48V for horseradish peroxidases (either in aerobic or anoxic conditions) and neutron activation analysis (NAA) for squash peroxidase, demonstrated a binding of vanadium to these enzymes in stoichiometric amounts. Electron paramagnetic resonance spectra of the eluted peaks for the former peroxidase indicated that vanadium is in the +5 oxidation state, but an equilibrium between V (V) and V (IV) in the assay conditions cannot be discarded. Although the inhibitory mechanism remains obscure, some hypotheses are considered. The potential implications that the inhibitory effect of vanadium might have on plant and animal metabolism are also discussed.