A biochemical rationale for the discrete behavior of nitroxyl and nitric oxide in the cardiovascular system

Abstract

The redox siblings nitroxyl (HNO) and nitric oxide (NO) have often been assumed to undergo casual redox reactions in biological systems. However, several recent studies have demonstrated distinct pharmacological effects for donors of these two species. Here, infusion of the HNO donor Angeli's salt into normal dogs resulted in elevated plasma levels of calcitonin gene-related peptide, whereas neither the NO donor diethylamine͞NONOate nor the nitrovasodilator nitroglycerin had an appreciable effect on basal levels. Conversely, plasma cGMP was increased by infusion of diethylamine͞NONOate or nitroglycerin but was unaffected by Angeli's salt. These results suggest the existence of two mutually exclusive response pathways that involve stimulated release of discrete signaling agents from HNO and NO. In light of both the observed dichotomy of HNO and NO and the recent determination that, in contrast to the O2͞O 2 ؊ couple, HNO is a weak reductant, the relative reactivity of HNO with common biomolecules was determined. This analysis suggests that under biological conditions, the lifetime of HNO with respect to oxidation to NO, dimerization, or reaction with O2 is much longer than previously assumed. Rather, HNO is predicted to principally undergo addition reactions with thiols and ferric proteins. Calcitonin gene-related peptide release is suggested to occur via altered calcium channel function through binding of HNO to a ferric or thiol site. The orthogonality of HNO and NO may be due to differential reactivity toward metals and thiols and in the cardiovascular system, may ultimately be driven by respective alteration of cAMP and cGMP levels.Angeli's salt ͉ superoxide dismutase ͉ heme protein ͉ cGMP ͉ calcitonin gene-related peptide D uring the last two decades, discussion of the chemistry of nitric oxide (NO) in biological systems has primarily focused on the nitrosylation of heme proteins such as soluble guanylyl cyclase and the production of reactive nitrogen oxide species (RNOS) (1-3). The RNOS literature has largely been concerned with nitrogen dioxide (NO 2 ), dinitrogen trioxide (N 2 O 3 ), and peroxynitrite (ONOO Ϫ ), which are formed through reaction with molecular oxygen or superoxide (O 2 Ϫ ) (4-6). Recently, however, there has been increased interest in the one-electron reduction product of NO, nitroxyl (HNO͞NO Ϫ ; nitrosyl hydride͞nitroxyl anion). Of particular note are studies suggesting that oxidation of L-arginine by NO synthase (NOS) leads to production of nitroxyl rather than NO under certain conditions (7-10). In this light, elucidation of the chemical biology of nitroxyl has acquired new importance.Comparisons of the toxicological and pharmacological properties of nitrogen oxide donor compounds have revealed that NO and HNO in general elicit distinct responses under a variety of biological conditions. In vitro, HNO reacts with O 2 to generate potent oxidizing species capable of cleaving DNA, thereby augmenting oxidative damage (3, 11). The RNOS formed by NO autoxidation do not cause these cellular a...

The potential of Angeli's salt to decrease nitric oxide scavenging by plasma hemoglobin

“…The inclusion of 200 μM GSH with the reaction of 50 μM AS and 100 μM oxyHb reduced metHb yield ( Figure 6D), but the effect of the GSH on the oxyHb reaction was not as great as it was on the metHb reaction. This result is consistent with published reaction rate constants for these reactions ( [85] and see Figure 1). The fact that addition of 50 μM AS to 100 μM oxyHb forms almost 100 μM metHb (Figure 3) suggests that Hb β-93 cysteine does not effectively compete with the heme reaction of oxyHb and AS derived HNO.…”

“…These results suggest that AS may hold therapeutic promise in the context of reducing NO scavenging by cell-free Hb in pathological conditions associated with hemolysis. Previous work has demonstrated the rapid reaction of HNO released from AS with oxyHb to form metHb [84][85][86]. The pathway likely involves co-oxidation of the iron atom and nitroxyl by the bound dioxygen to yield metHb, NO and an equivalent of hydrogen peroxide (reaction 2, Figure 1) [84], (7) The NO formed can then react with another oxyHb to form metHb and nitrate (reaction 3, Figure 1), giving the relation of Equation 4 where one AS derived HNO converts two oxyHb to two metHb [84].…”

“…The rate constants for the reaction of HNO with oxyHb (about 10 7 M −1 s −1 ) [85] is comparable to NO, and thus it may preferentially react with cell-free Hb compared to RBC encapsulated Hb in a similar way as NO. In addition, HNO reacts with ferric hemes to form an iron nitrosyl with a rate constant of about 10 6 M −1 s −1 [85,86], so we reasoned that HNO would also form iron nitrosyl Hb when reacted with oxyHb.…”

“…In addition, HNO reacts with ferric hemes to form an iron nitrosyl with a rate constant of about 10 6 M −1 s −1 [85,86], so we reasoned that HNO would also form iron nitrosyl Hb when reacted with oxyHb. A reaction scheme for Angeli's salt (AS) and various Hb forms is presented in Figure 1.…”

How widespread is stable protein S-nitrosylation as an end-effector of protein regulation?

“…Formation of HNO adds additional complexity, as it also reacts with GSH (6 × 10 6 M −1 s − [52]) to produce the same products without the need for catalysis by GSNOR (eq. 7-9).…”