N -Acetylcysteine Does Not Influence the Activity of Endothelium-Derived Relaxing Factor In Vivo

Abstract

Nitric oxide forms complexes with an array of biomolecular carriers that retain biological activity. This reactivity of nitric oxide in physiological systems has led to some dispute as to whether endothelium-derived relaxing factors nitric oxide or a closely related adduct thereof, such as a nitrosothiol. In vitro bioassays used to address this question are limited by the exclusion of biological thiols that are requisite for nitrosothiol formation. Thus, the purpose of this study was to obtain insight into the identity of endothelium-derived relaxing factor in vivo. We reasoned that if endothelium-derived relaxing factor in nitric oxide, infusion of physiological concentrations of thiol would potentiate its bioactivity by analogy with effects seen in vitro, whereas nitrosothiol would be resistant to such modulation. We used venous-occlusion plethysmography to study forearm blood flow in normal subjects. Methacholine (0.3 to 10 micrograms/min) and nitroglycerin (1 to 30 micrograms/min) were infused via the brachial artery to elicit endothelium-dependent and endothelium-independent vasodilation, respectively. Dose-response determinations were made for each drug before and after an intra-arterial infusion of the reduced thiol, N-acetylcysteine, at rates estimated to achieve a physiological concentration of 1 mmol/L. Methacholine increased forearm blood flow in a dose-dependent manner. Infusion of N-acetylcysteine did not change the sensitivity (ED50, 1.7 versus 1.7 micrograms/min, P = NS) or maximal response to methacholine. In contrast, thiol increased the sensitivity to nitroglycerin (ED50, 4.7 versus 2.8 micrograms/min, P < .01). Thus, conflicting with reports in vitro, thiol does not modulate endothelium-derived relaxing factor responses in vivo. These data indicate that sulfhydryl groups are not a limiting factor for endothelium-derived relaxing factor responses in forearm resistance vessels in normal humans and are in keeping with reports that nitrosothiol contributes to endothelium-derived relaxing factor bioactivity in plasma and vascular smooth muscle. Potentiation of the effects of nitroglycerin by N-acetylcysteine can be attributed to its enhanced biotransformation to an endothelium-derived relaxing factor equivalent, such as nitrosothiol. These observations support the notion of an equilibrium between nitric oxide and nitrosothiol in biological systems that may be influenced by redox state.

Decreased aortic glutathione levels may contribute to impaired nitric oxide‐induced relaxation in hypercholesterolaemia

“…This may be due to the fact that in the presence of oxygen, superoxide anion, or trace metals, NO may be oxidized and combine with GSH to form GSNO (Stamler, 1994), and increased formation of this longer lasting form of NO might explain the enhancement in the response observed. If as previously suggested, endothelium‐derived NO is itself carried by low molecular weight thiols (Myers et al ., 1990; Creager et al ., 1997), this may explain why ACh‐induced relaxation was not enhanced by increasing GSH levels. It is not clear why GSH did not change SNP‐induced relaxation, but it might be that NO released from SNP does not form nitrosothiols.…”

“…Glutathione ( L ‐γ‐glutamyl‐ L ‐cysteinylglycine, GSH) is the most abundant low molecular weight thiol in vascular tissue (Vita et al ., 1998), and may affect the bioactivity of NO. S‐nitrosothiols may have a role as endothelium‐derived relaxing factors (Myers et al ., 1990; Creager et al ., 1997), and GSH is the precursor of the nitrosothiol, S‐nitrosoglutathione (GSNO). GSNO can transnitrosate protein thiols, possibly changing protein function (Stamler, 1994).…”

Role of Oxidant Stress in Endothelial Dysfunction Produced by Experimental Hyperhomocyst(e)inemia in Humans

“…25,26 Plasma methionine concentrations increased by almost 30-fold compared with an increase of only 4-fold in plasma homocysteine (Figure 1). However, endothelial function was not significantly impaired 1 hour after administration of oral methionine, when plasma methionine was already markedly elevated but plasma homocysteine concentrations were only 11 mol/L.…”

Nitric oxide, prostanoid and non‐NO, non‐prostanoid involvement in acetylcholine relaxation of isolated human small arteries

“…The relatively modest eect of the NO scavenger oxyhaemoglobin on SNAP relaxation has also been observed by others (Kemp & Cocks, 1997), and may be related to possible intracellular release of NO. SNAP is a nitrosothiol and both in vitro and in vivo studies suggest that these substances could play a role as endothelium derived mediators (Myers et al, 1990;Creager et al, 1997). However, the inhibitors of soluble guanylyl cyclase and protein kinase G abolished SNAP induced relaxations, while they hardly changed the ACh relaxation.…”