The impact of metal catalysis on protein tyrosine nitration by peroxynitrite

In a series of heme and non-heme proteins the nitration of tyrosine residues was assessed by complete pronase digestion and subsequent HPLC-based separation of 3-nitrotyrosine. Bolus addition of peroxynitrite caused comparable nitration levels in all tested proteins. Nitration mainly depended on the total amount of tyrosine residues as well as on surface exposition. In contrast, when superoxide and nitrogen monoxide (NO) were generated at equal rates to yield low steady-state concentrations of peroxynitrite, metal catalysis seemed to play a dominant role in determining the sensitivity and selectivity of peroxynitrite-mediated tyrosine nitration in proteins. Especially, the heme-thiolate containing proteins cytochrome P450(BM-3) (wild type and F87Y variant) and prostacyclin synthase were nitrated with high efficacy. Nitration by co-generated NO/O(2)(-) was inhibited in the presence of superoxide dismutase. The NO source alone only yielded background nitration levels. Upon changing the NO/O(2)(-) ratio to an excess of NO, a decrease in nitration in agreement with trapping of peroxynitrite and derived radicals by NO was observed. These results clearly identify peroxynitrite as the nitrating species even at low steady-state concentrations and demonstrate that metal catalysis plays an important role in nitration of protein-bound tyrosine.

Upregulation of phosphodiesterase 1A1 expression is associated with the development of nitrate tolerance

BACKGROUND

The efficacy of nitroglycerin (NTG) as a vasodilator is limited by tolerance, which develops shortly after treatment begins. In vascular smooth muscle cells (VSMCs), NTG is denitrated to form nitric oxide (NO), which activates guanylyl cyclase and generates cGMP. cGMP plays a key role in nitrate-induced vasodilation by reducing intracellular Ca(2+) concentration. Therefore, one possible mechanism for development of nitrate tolerance would be increased activity of the cGMP phosphodiesterase (PDE), which decreases cGMP levels.

METHODS AND RESULTS

To test this hypothesis, rats were made tolerant by continuous infusion of NTG for 3 days (10 microgram kg(-1). min(-1) SC) with an osmotic pump. Analysis of PDE activities showed an increased function of Ca(2+)/calmodulin (CaM)-stimulated PDE (PDE1A1), which preferentially hydrolyzes cGMP after NTG treatment. Western blot analysis for the Ca(2+)/CaM-stimulated PDE revealed that PDE1A1 was increased 2.3-fold in NTG-tolerant rat aortas. Increased PDE1A1 was due to mRNA upregulation as measured by relative quantitative reverse transcription-polymerase chain reaction. The PDE1-specific inhibitor vinpocetine partially restored the sensitivity of the tolerant vasculature to subsequent NTG exposure. In cultured rat aortic VSMCs, angiotensin II (Ang II) increased PDE1A1 activity, and vinpocetine blocked the effect of Ang II on decrease in cGMP accumulation.

CONCLUSIONS

Induction of PDE1A1 in nitrate-tolerant vessels may be one mechanism by which NO/cGMP-mediated vasodilation is desensitized and Ca(2+)-mediated vasoconstriction is supersensitized. Inhibiting PDE1A1 expression and/or activity could be a novel therapeutic approach to limit nitrate tolerance.

Mechanisms underlying endothelial dysfunction in diabetes mellitus

Incubation of endothelial cells in vitro with high concentrations of glucose activates protein kinase C (PKC) and increases nitric oxide synthase (NOS III) gene expression as well as superoxide production. The underlying mechanisms remain unknown. To address this issue in an in vivo model, diabetes was induced with streptozotocin in rats. Streptozotocin treatment led to endothelial dysfunction and increased vascular superoxide production, as assessed by lucigenin- and coelenterazine-derived chemiluminescence. The bioavailability of vascular nitric oxide (as measured by electron spin resonance) was reduced in diabetic aortas, although expression of endothelial NOS III (mRNA and protein) was markedly increased. NOS inhibition with N:(G)-nitro-L-arginine increased superoxide levels in control vessels but reduced them in diabetic vessels, identifying NOS as a superoxide source. Similarly, we found an activation of the NADPH oxidase and a 7-fold increase in gp91(phox) mRNA in diabetic vessels. In vitro PKC inhibition with chelerythrine reduced vascular superoxide in diabetic vessels, whereas it had no effect on superoxide levels in normal vessels. In vivo PKC inhibition with N:-benzoyl-staurosporine did not affect glucose levels in diabetic rats but prevented NOS III gene upregulation and NOS-mediated superoxide production, thereby restoring vascular nitric oxide bioavailability and endothelial function. The reduction of superoxide in vitro by chelerythrine and the normalization of NOS III gene expression and reduction of superoxide in vivo by N:-benzoyl-staurosporine point to a decisive role of PKC in mediating these phenomena and suggest a therapeutic potential of PKC inhibitors in the prevention or treatment of vascular complications of diabetes mellitus. The full text of this article is available at http://www.circresaha.org.

Spin trapping of vascular nitric oxide using colloid Fe(II)-diethyldithiocarbamate

Currently available EPR spin-trapping techniques are not sensitive enough for quantification of basal vascular nitric oxide (NO) production from isolated vessels. Here we demonstrate that this goal can be achieved by the use of colloid Fe(DETC)(2). Rabbit aortic or venous strips incubated with 250 microM colloid Fe(DETC)(2) exhibited a linear increase in tissue-associated NO-Fe(DETC)(2) EPR signal during 1 h. Removal of endothelium or addition of 3 mM N(G)-nitro-l-arginine methyl ester (L-NAME) inhibited the signal. The basal NO production was estimated as 5.9 +/- 0.5 and 8.3 +/- 2.1 pmol/min/cm(2) in thoracic aorta and vena cava, respectively. Adding sodium nitrite (10 microM) or xanthine/xanthine oxidase in the incubation medium did not modify the intensity of the basal NO-Fe(DETC)(2) EPR signal. Reducing agents were not required with this method and superoxide dismutase activity was unchanged by the Fe(DETC)(2) complex. We conclude that colloid Fe(DETC)(2) may be a useful tool for direct detection of low amounts of NO in vascular tissue.

Converting enzyme inhibition by enalapril in experimental heart failure

We analyzed the effect of enalapril (0.1 mg/kg p.o. twice daily) on plasma electrolytes, urea, and creatinine in low cardiac output failure. In 14 male dogs implanted with chronic instrumentation, tachycardia was induced by ventricular pacing (265 impulses/min., 10-14 days). In 7 untreated dogs, pacing progressively lowered aortic flow by 44% and induced hyponatremia and elevations of plasma urea, creatinine, and potassium. Treatment with enalapril (n = 7) during pacing reduced the decrease in aortic flow by 33% and prevented changes in plasma urea, potassium, and sodium. We conclude that this is due to enalapril-induced retardation of heart failure progression.