Thiol-independent stimulation of soluble guanylate cyclase by NO-containing compounds

Treatment with 1,2,3,4-tetrahydroisoquinolone affects the levels of nitric oxide, S-nitrosothiols, glutathione and the enzymatic activity of γ-glutamyl transpeptidase in the dopaminergic structures of rat brain

Depletion of glutathione (GSH), nitrosative stress and chronic intoxication with some neurotoxins have been postulated to play a major role in the pathogenesis of Parkinson's disease. This study aimed to examine the effects of acute and chronic treatments with 1,2,3,4-tetrahydroisoquinoline (TIQ), an endo-/exogenous substance suspected of producing Parkinsonism in human, on the levels of nitric oxide (NO), S-nitrosothiols and glutathione (GSH) in the whole rat brain and in its dopaminergic structures. TIQ administered at a dose of 50 mg/kg i.p. significantly increased the tissue concentrations of NO and GSH in the substantia nigra (SN), striatum (STR) and cortex (CTX) of rats receiving this compound both acutely and chronically. Moreover, it decreased the level of oxidized glutathione (GSSG) and enhanced GSH:GSSG ratio affecting in this way the redox state of brain cells. TIQ also increased the level of S-nitrosothiols when measured in the whole rat brain and CTX, although it markedly decreased their level in the STR after both treatments. Inhibition of the constitutive NO synthase by l-NAME in the presence of TIQ caused decreases in GSH and S-nitrosothiol levels in the brain. The latter effect shows that the TIQ-mediated increases in GSH and S-nitrosothiol concentrations were dependent on the enhanced NO level. The above-described results suggest that TIQ can act as a modulator of GSH, NO and S-nitrosothiol levels but not as a parkinsonism-inducing agent in the rat brain.

Dominant negative mutants of guanylyl cyclase: probes for global functions and intramolecular mechanisms

Dominant negative mutants are unique tools to define functions of a protein, not only within complex cellular and organismal contexts, but also mechanistically within a protein. Guanylyl cyclases are amenable to studies with dominant negative mutants, with their own sets of opportunities for insight and pitfalls to overcome. Membrane and soluble forms of guanylyl cyclase represent self-contained signal transduction modules that recognize, transduce, and amplify an external signal to give a carefully controlled response. Beginning with recognition of peptide hormones versus nitric oxide, membrane and soluble guanylyl cyclases are considerably different, except that their catalytic domains are closely related. Studies on these catalytic domains and their counterparts in adenylyl cyclases have raised an integral question of whether one or two domains form a catalytic site, which remains unresolved. Regardless of which model is correct, guanylyl cyclases appear to require an oligomeric state to function properly. The inferred relationship between protein-protein interaction and function is the basis for developing dominant negative mutants, which can be designed without prior structural information. Soluble guanylyl cyclases exist in a heterodimeric state, whereas membrane guanylyl cyclases are homodimeric, or possibly higher-order oligomers. These properties dictate that dominant negative mutants of membrane and soluble guanylyl cyclases be approached in fundamentally different ways, with regard to their design, their functional consequences, and their limitations. Using dominant negative mutants as specific inhibitors in complex systems, such as transgenic animals, represents a significant advance, and continuing improvements are just an inkling of the extraordinary potential of this approach. For example, the function of a protein can be obscured because it is expressed in multiple cell types; by restricting its pattern of expression, a cell-specific promoter, coupled to a dominant negative mutant, can pinpoint this function. As more sophisticated methods are developed, dominant negative mutants will provide additional opportunities to unveil new regulatory mechanisms, new signaling pathways, or even new therapeutic approaches.

Peroxynitrite-induced accumulation of cyclic GMP in endothelial cells and stimulation of purified soluble guanylyl cyclase. Dependence on glutathione and possible role of S-nitrosation

Peroxynitrite (ONOO-) is widely recognized as mediator of NO toxicity, but recent studies have indicated that this compound may also have physiological activity and induce vascular relaxation as well as inhibition of platelet aggregation. We found that ONOO- induced a pronounced increase in endothelial cyclic GMP levels, and that this effect was significantly attenuated by pretreatment of the cells with GSH-depleting agents. In the presence of 2 mM GSH, ONOO- stimulated purified soluble guanylyl cyclase with a half-maximally effective concentration of about 20 microM. In contrast to the NO donor 2,2-Diethyl-1-nitroso-oxyhydrazine sodium salt (DEA/NO), ONOO- was completely inactive in the absence of GSH, indicating that thiol-mediated bioactivation of ONOO- is involved in enzyme stimulation. Studies on the reaction between ONOO- and GSH revealed that about 1% of ONOO- was non-enzymatically converted to S-nitrosoglutathione. The authentic nitrosothiol was found to be stable in solution, but slowly decomposed in the presence of GSH. GSH-induced decomposition of S-nitrosoglutathione was apparently catalyzed by trace metals and was accompanied by a sustained release of NO and a 40-100-fold increase in its potency to stimulate purified soluble guanylyl cyclase. Our data suggest that the biologic activity of ONOO- involves S-nitrosation of cellular thiols resulting in NO-mediated cyclic GMP accumulation.

NO accounts completely for the oxygenated nitrogen species generated by enzymic L-arginine oxygenation

We have assessed the stoichiometry of the nitric oxide (NO) synthase reaction by using a novel e.p.r. technique. NO generated by crude and partially purified NO synthase from endothelial cells and Escherichia coli-lipopolysaccharide-activated macrophages was trapped by a ferrous diethyldithiocarbamate complex dispersed in yeast. The paramagnetic ferrous mononitrosyl dithiocarbamate complex formed exhibited a characteristic e.p.r. signal at g perpendicular = 2.035 and g parallel = 2.02 with a triplet hyperfine structure (hfs) at g perpendicular. NO, 3-morpholinosydnonimine and S-nitroso-L-cysteine, but not nitrite or hydroxylamine, generated a similar e.p.r. signal. NO generated by NO synthase and by SIN-1 accumulated at a constant rate for 1 h, as measured by continuous e.p.r. registration at 37 degrees C. The formation of e.p.r.-detectable NO by NO synthases was inhibited by NG-nitro-L-arginine. Incubation with [15N]NG-L-arginine caused an e.p.r. signal with doublet hfs, indicating that the nitrosyl nitrogen derived exclusively from the guanidino nitrogen. The amount of NO generated by NO synthase as measured by e.p.r. technique was compared with formation of L-[3H]citrulline from L-[3H]arginine. NO and L-citrulline were detected at a 1:1 ratio with both NO synthase preparations. GSH and thiol depletion did not significantly affect NO synthase activity, excluding S-nitrosothiols as intermediates in the NO synthase reaction. We conclude that NO fully accounts for the immediate oxygenated nitrogen species derived from the enzymic oxygenation of L-arginine.

L-cysteine augments the vasorelaxation induced by sodium nitrite and SIN-1 but not that due to acetylcholine

The effects of 1 mM L-cysteine on sodium nitrite-, 3-morpholinosydnonimine (SIN-1)- and acetylcholine-induced relaxation and cyclic GMP accumulation were studied in isolated noradrenaline-precontracted rat mesenteric arterial rings. L-Cysteine augmented the relaxation and cyclic GMP increase induced by sodium nitrate and SIN-1 but not those induced by acetylcholine. The effects of L-cysteine on relaxation were independent of the presence of intact endothelium. The results suggest that L-cysteine protects exogenously released nitric oxide.