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Erythrocytes are peculiar cells aimed at the delivery of oxygen and nitric oxide to the periphery and carbon dioxide to the lungs. In addition, they also exert, under physiological conditions, a scavenging activity towards reactive oxygen and nitrogen species often over-produced in morbidity states, e.g. in inflamed tissues. Their deformability is essential for their circulation, specifically in small blood vessels, and this is an important pre-requisite for such vascular "antioxidant" functions. On the other hand, if the erythrocyte undergoes changes in its redox status, i.e. is not capable of counteracting the pro-oxidant status of the microenvironment, it becomes a source of reactive species and, consequently, its typical structural and functional features are lost. More importantly, the oxidatively modified red cell increases its aggregability and adhesiveness to the endothelium and to other blood cells, thus contributing to vascular damage. In line with recent data from the literature, erythrocytes can be proposed as bioindicators of progression in chronic or acute diseases characterized, as a hallmark, by oxidative alterations.

Peroxynitrite, the product of the reaction between nitric oxide and superoxide anion, is able to nitrate protein tyrosines. If this modification occurs on phosphotyrosine kinase substrates, it can down-regulate cell signaling. We investigated the effects of peroxynitrite on band 3-mediated signal transduction of human erythrocytes. Peroxynitrite treatment induced two different responses. At low concentrations (10-100 microM) it stimulated a metabolic response, leading to 1) a reversible inhibition of phosphotyrosine phosphatase activity, 2) a rise of tyrosine phosphorylation in the 22K cytoplasmic domain of band 3, 3) the release of glyceraldehyde 3-phosphate dehydrogenase from the membrane, and 4) the enhancement of lactate production. At high concentrations (200-1000 microM), peroxynitrite induced 1) cross-linking of membrane proteins, 2) inhibition of band 3 tyrosine phosphorylation, 3) nitration of tyrosines in the 22K cytoplasmic domain of band 3, 4) binding of hemoglobin to the membrane, 5) irreversible inhibition of phosphotyrosine kinase activity, 6) massive methemoglobin production, and 7) irreversible inhibition of lactate production. Our results demonstrate that at concentrations that could conceivably be achieved in vivo (10-100 microM), peroxynitrite behaves like other oxidants, i.e., it stimulates band 3 tyrosine phosphorylation and increases glucose metabolism. Thus, one plausible physiologic effect of peroxynitrite is the up-regulation of signaling through the reversible inhibition of phosphotyrosine phosphatase activity. At high concentrations of peroxynitrite, the tyrosine phosphorylation ceases in parallel with the nitration of band 3 tyrosines, but at these concentrations phosphotyrosine kinase activity and glycolysis are also irreversibly inhibited. Thus, at least in red blood cells, the postulated down-regulation of signaling by peroxynitrite cannot merely be ascribed to the nitration of tyrosine kinase targets.

Peroxynitrite-mediated oxidative chemistry is currently the subject of intense investigation owing to the toxic side effects associated with nitric oxide overproduction. Using direct electron spin resonance spectroscopy (ESR) at 37 degrees C, we observed that in human erythrocytes peroxynitrite induced a long-lived singlet signal at g = 2.004 arising from hemoglobin. This signal was detectable in oxygenated red blood cells and in purified oxyhemoglobin but significantly decreased after deoxygenation. The formation of the g = 2.004 radical required the presence of CO2 and pH values higher than the pKa of peroxynitrous acid (pKa = 6.8), indicating the involvement of a secondary oxidant formed in the interaction of ONOO- with CO2. The g = 2.004 radical yield leveled off at a 1:1 ratio between peroxynitrite and oxyhemoglobin, while CO-hemoglobin formed less radical and methemoglobin did not form the radical at all. These results suggest that the actual oxidant is or is derived from the ONOOCO2- adduct interacting with oxygenated FeII-heme. Spin trapping with 2-methyl-2-nitrosopropane (MNP) of the g = 2.004 radical and subsequent proteolytic digestion of the MNP/hemoglobin adduct revealed the trapping of a tyrosyl-centered radical(s). A similar long-lived unresolved g = 2.004 singlet signal is a common feature of methemoglobin/H2O2 and metmyoglobin/H2O2 systems. We show by spin trapping that these g = 2.004 signals generated by H2O2 also indicated trapping of radicals centered on tyrosine residues. Analysis of visible spectra of hemoglobin treated with peroxynitrite revealed that, in the presence of CO2, oxyhemoglobin was oxidized to a ferryl species, which rapidly decayed to lower iron oxidation states. The g = 2.004 radical may be an intermediate formed during ferrylhemoglobin decay. Our results describe a new pathway of peroxynitrite-dependent hemoglobin oxidation of dominating importance in CO2-containing biological systems and identify the g = 2.004 radical(s) formed in the process as tyrosyl radical(s).

Changes in the oxidative status of erythrocytes can reduce cell lifetime, oxygen transport, and delivery capacity to peripheral tissues and have been associated with a plethora of human diseases. Among reactive oxygen and nitrogen species of importance in red blood cell (RBC) homeostasis, superoxide and nitric oxide radicals play a key role. In the present work, we evaluated subcellular effects induced by peroxynitrite, the product of the fast reaction between superoxide and nitric oxide. Peroxynitrite induced 1) oxidation of oxyhemoglobin to methemoglobin, 2) cytoskeleton rearrangement, 3) ultrastructural alterations, and 4) altered expression of band-3 and decreased expression of glycophorin A. With respect to control cells, this occurred in a significantly higher percentage of human RBC (approximately 40%). The presence of antioxidants inhibited these modifications. Furthermore, besides these senescence-associated changes, other important modifications, absent in control RBC and usually associated with apoptotic cell death, were detected in a small but significant subset of peroxynitrite-exposed RBC (approximately 7%). Active protease cathepsin E and mu-calpain increased; activation of caspase 2 and caspase 3 was detected; and phosphatidylserine externalization, an early marker of apoptosis, was observed. Conversely, inhibition of cathepsin E, mu-calpain, as well as caspase 2 and 3 by specific inhibitors resulted in a significant impairment of erythrocyte "apoptosis" Altogether, these results indicate that peroxynitrite, a milestone of redox-mediated damage in human pathology, can hijack human RBC toward senescence and apoptosis by a mechanism involving both cysteinyl and aspartyl proteases.

Peroxynitrite, the product of the radical-radical reaction between nitric oxide and superoxide anion, is a potent oxidant involved in tissue damage in neurodegenerative disorders. We investigated the modifications induced by peroxynitrite in tyrosine residues of proteins from synaptosomes. Peroxynitrite treatment (> or =50 microM) induced tyrosine nitration and increased tyrosine phosphorylation. Synaptophysin was identified as one of the major nitrated proteins and pp60src kinase as one of the major phosphorylated substrates. Further fractionation of synaptosomes revealed nitrated synaptophysin in the synaptic vesicles, whereas phosphorylated pp60src was enriched in the postsynaptic density fraction. Tyrosine phosphorylation was increased by treatment with 50-500 microM peroxynitrite and decreased by higher concentrations, suggesting a possible activation/inactivation of kinases. Immunocomplex kinase assay proved that peroxynitrite treatment of synaptosomes modulated the pp60src autophosphorylation activity. The addition of bicarbonate (CO2 1.3 mM) produced a moderate enhancing effect on some nitrated proteins but significantly protected the activity of pp60src against peroxynitrite-mediated inhibition so that at 1 mM peroxynitrite, the kinase was still more active than in untreated synaptosomes. The phosphotyrosine phosphatase activity of synaptosomes was inhibited by peroxynitrite (> or =50 microM) but significantly protected by CO2. Thus, the increase of phosphorylation cannot be attributed to peroxynitrite-mediated inhibition of phosphatases. We suggest that peroxynitrite may regulate the posttranslational modification of tyrosine residues in pre- and postsynaptic proteins. Identification of the major protein targets gives insight into the pathways possibly involved in neuronal degeneration associated with peroxynitrite overproduction.