Differential effect of L-histidine in human lymphocytes damaged by different oxygen radical producing systems

Oxidative stress to human lymphocytes by xanthine oxidoreductase activity

The in vitro toxicity of the reactive oxygen species generating enzyme xanthine oxidoreductase (XOR) to human peripheral blood lymphocytes was studied after stimulation with phytohaemoagglutinin or anti-CD3/CD28 antibodies. Apoptosis and necrosis were induced by the XOR/hypoxanthine system in a time- and concentration-dependent manner. CD8+ lymphocytes showed a higher sensitivity than CD4+ cells to the XOR/hypoxanthine system. The occurrence of apoptosis was demonstrated by annexin-V binding to injured cell membrane, which was the most precocious alteration observed, followed by the increment of transglutaminase activity, which was significant at the lowest XOR concentration used. Nuclear damage was assessed by the increased hypodiploid nuclei and by DNA migration on gel electrophoresis, which turned to an apoptotic pattern before the occurrence of cell membrane necrotic lesions. Apoptosis was induced by XOR activity proportionally to substrate concentration and was prevented by the competitive enzyme inhibitor, allopurinol. The hydrogen peroxide scavenging enzyme, catalase, gave a higher protection than superoxide dismutase from the toxicity caused by the XOR/hypoxanthine system. Necrosis occurs in a variable percentage indicating that reactive oxygen species may trigger both apoptosis and necrosis in proliferating human lymphocytes, mostly depending on XOR concentration.

Disruption of the uncoupling protein-2 gene in mice reveals a role in immunity and reactive oxygen species production

The gene Ucp2 is a member of a family of genes found in animals and plants, encoding a protein homologous to the brown fat uncoupling protein Ucp1 (refs 1-3). As Ucp2 is widely expressed in mammalian tissues, uncouples respiration and resides within a region of genetic linkage to obesity, a role in energy dissipation has been proposed. We demonstrate here, however, that mice lacking Ucp2 following targeted gene disruption are not obese and have a normal response to cold exposure or high-fat diet. Expression of Ucp2 is robust in spleen, lung and isolated macrophages, suggesting a role for Ucp2 in immunity or inflammatory responsiveness. We investigated the response to infection with Toxoplasma gondii in Ucp2-/- mice, and found that they are completely resistant to infection, in contrast with the lethality observed in wild-type littermates. Parasitic cysts and inflammation sites in brain were significantly reduced in Ucp2-/- mice (63% decrease, P<0.04). Macrophages from Ucp2-/- mice generated more reactive oxygen species than wild-type mice (80% increase, P<0.001) in response to T. gondii, and had a fivefold greater toxoplasmacidal activity in vitro compared with wild-type mice (P<0.001 ), which was absent in the presence of a quencher of reactive oxygen species (ROS). Our results indicate a role for Ucp2 in the limitation of ROS and macrophage-mediated immunity.

Oxidative stress induces age-dependent changes in lymphocyte protein synthesis and second messenger levels

Cumulative damage in cells from aged people could lead to a greater fragility against acute oxidative stress. The effects of acute oxidative stress on cell viability, cAMP and cGMP concentrations, and protein synthesis rates were studied in lymphocytes from 25 young and 26 elderly subjects. Lymphocytes were exposed to stress by hydrogen peroxide 25 micromol/l and incubated for 18 hours. Cell viability after stress was lower (p<0.0001, Student's t test) in cells from the elderly (63.4%) than in cells from the young donors (73.2%). The protein synthesis rate was also lower after stress (p<0.04, Mann-Whitney U test) in cells from the elderly (47.3% vs. non-stressed cells), than in cells from the young (82.19% vs. non-stressed cells). After oxidative stress, cAMP and cGMP concentrations showed no significant changes in cells from young subjects; there were, however, significant decreases in these cyclic nucleotides in cells from the elderly (p<0.008 for both nucleotides, paired Student's t test). There were no differences in basal cAMP or cGMP levels between the two groups. These results show that mortality and metabolic changes due to oxidative stress are greater in lymphocytes proceeding from elderly subjects than in those from young subjects.

The biological activity of hydrogen peroxide. VI. Mechanism of the enhancing effects of L-histidine: the role of the formation of a histidine-peroxide adduct and membrane transport

Further details of the mechanism of the enhancing effects of L-histidine (L-His) on the clastogenic activities of hydrogen peroxide (H2O2) were investigated. The L-His-H2O2 adduct was prepared and its physicochemical properties and biological activities were compared with those of a mixture of L-His plus H2O2 and of H2O2 alone. When the stabilities of the three test samples against glucose were determined in terms of residual H2O2 content in solutions of various pH values over the course of 11 days, the adduct was found to be more stable than H2O2 alone and very similar in terms of stability to the mixture. The almost equivalent stability of the adduct and the mixture suggested formation of the adduct in the mixture even though the interaction between L-His and H2O2 in solution seems, from 13C-NMR analysis, to be rather weak. In cell-free DNA after lysis of cell membranes, the induction of single-strand breaks (SSB) by the adduct and by the mixture was less effective than by H2O2 alone. These results contrast with previous results obtained in intact cells (Oya et al., 1992) and demonstrate the indispensability of the cell membrane for the enhancing effects of L-His. In the presence of inhibitors of the active transport of L-His, namely, 10 different neutral amino acids, effective suppression of the clastogenic activity of the adduct and of the mixture was observed, whereas four acidic and basic amino acids had no effect. Thus, the participation of active transport in the enhancing effects of L-His was apparent. The formation of the adduct of L-His with H2O2 brings about the stabilization or reduces the reactivity of H2O2 and, as a result, the induction of SSB is prevented to some extent in cell-free DNA systems. By contrast, in a cellular system, the accumulation of the adduct in cells by active transport is potentiated by the enhancing effect of L-His, although the mediation of some factors that can generate hydroxyl radicals (*OH) from the adduct in cells must be postulated.

Inhibition of poly(ADP-ribose) polymerization preserves the glutathione pool and reverses cytotoxicity in hydrogen peroxide-treated lymphocytes

DNA damage caused by oxygen radicals activates poly(ADP-ribosyl) polymerase (pADPRP), a nuclear enzyme that utilizes NAD+ as substrate. It has been demonstrated that pharmacological inactivation of pADPRP rescues human lymphocytes damaged by oxygen radicals, but not those damaged by equitoxic doses of ionizing radiation. In the present paper we demonstrate that the NAD+ pool decreases after both damaging treatments and is preserved in a similar fashion by pADPRP inhibition. On the contrary, the ATP pool, cell energy charge and reduced thiols are decreased only by the administration of oxygen radicals, and are preserved if poly(ADP)ribosylation is inhibited. In fact, treatment with oxidant agents depletes the cell energy pools owing to the simultaneous demands of the glutathione (GSH)/NADPH cycle and pADPRP-driven NAD+ consumption, while in irradiated cells only the latter mechanism operates. We suggest that, when pADPRP is inhibited, enough energy is available for the preservation of cell thiols, thereby allowing oxidant-treated cells to survive and undergo mitosis. Thus, GSH and energy shortage appear to be the main cause of cell death in oxidant-injured cells.