tetrahydroneopterin and Reperfusion-Injury

6R-5,6,7,8-tetrahydro-L-biopterin (6R-BH4) is a cofactor for endothelial nitric oxide synthase but also has antioxidant properties. Its stereo-isomer 6S-5,6,7,8-tetrahydro-L-biopterin (6S-BH4) and structurally similar pterin 6R,S-5,6,7,8-tetrahydro-D-neopterin (NH4) are also antioxidants but have no cofactor function. When endothelial nitric oxide synthase is 6R-BH4-deplete, it synthesizes superoxide rather than nitric oxide. Reduced nitric oxide bioavailability by interaction with reactive oxygen species is implicated in endothelial dysfunction (ED). 6R-BH4 corrects ED in animal models of ischemia reperfusion injury (IRI) and in patients with cardiovascular risks. It is uncertain whether the effect of exogenous 6R-BH4 on ED is through its cofactor or antioxidant action.. In healthy volunteers, forearm blood flow was measured by venous occlusion plethysmography during intra-arterial infusion of the endothelium-dependent vasodilator acetylcholine, or the endothelium-independent vasodilator glyceryl trinitrate, before and after IRI. IRI reduced plasma total antioxidant status (P=0.03) and impaired vasodilatation to acetylcholine (P=0.01), but not to glyceryl trinitrate (P=0.3). Intra-arterial infusion of 6R-BH4, 6S-BH4 and NH4 at approximately equimolar concentrations prevented IRI.. IRI causes ED associated with increased oxidative stress that is prevented by 6R-BH4, 6S-BH4, and NH4, an effect mediated perhaps by an antioxidant rather than cofactor function. Regardless of mechanism, 6R-BH4, 6S-BH4, or NH4 may reduce tissue injury during clinical IRI syndromes.

Topics: Acetylcholine; Adult; Antioxidants; Biopterins; Coenzymes; Dose-Response Relationship, Drug; Endothelium, Vascular; Female; Forearm; Humans; Male; Nitric Oxide; Nitric Oxide Synthase; Oxidative Stress; Regional Blood Flow; Reperfusion Injury; Stereoisomerism; Time Factors; Vasodilation; Vasodilator Agents

Tetrahydrobiopterin has been shown to efficiently abrogate ischemia reperfusion injury (IRI). However, it is unclear, whether its beneficial action relies on cofactor activity of one of the five known tetrahydrobiopterin-dependent reactions or on its antioxidative capacity. We therefore compared tetrahydrobiopterin with the pterin derivate tetrahydroneopterin (similar biochemical properties, but no nitric oxide synthase cofactor activity) and the antioxidants vitamin C and 5-methyltetrahydrofolate. Donor mice were pretreated with tetrahydrobiopterin, tetrahydroneopterin, vitamin C, or 5-methyltetrahydrofolate. Pancreatic grafts were subjected to 16-h cold ischemia time and implanted in syngeneic recipients. Untreated and nontransplanted animals served as controls. Following 2-h reperfusion, microcirculation was analyzed by intravital fluorescence microscopy. Graft damage was assessed by histology and nitrotyrosine immunostaining, and tetrahydrobiopterin levels were determined by HPLC. Recipient survival served as ultimate readout. Prolonged cold ischemia time resulted in microcirculatory breakdown. Only tetrahydrobiopterin pretreatment succeeded to preserve the capillary net, whereas all other compounds showed no beneficial effects. Along with increased intragraft tetrahydrobiopterin levels during recovery and implantation, only tetrahydrobiopterin pretreatment led to significant reduction of IRI-related parenchymal damage enabling recipient survival. These results show a striking superiority of tetrahydrobiopterin in preventing lethal IRI compared with related compounds and suggest nitric oxide synthases as treatment target.

Topics: Animals; Antioxidants; Ascorbic Acid; Biopterins; Cold Ischemia; Immunohistochemistry; Ischemia; Liver; Male; Mice; Mice, Inbred C57BL; Microcirculation; Microscopy, Confocal; Nitric Oxide; Organ Preservation; Pancreas; Pancreas Transplantation; Reperfusion Injury; Tetrahydrofolates; Time Factors

We investigated the effects of D-neopterin (NP) and its reduced form, 5,6,7,8-tetrahydro-D-neopterin (NPH4), in two models of ischemia-reperfusion injury, i.e., ischemic paw edema in mice and gastric ischemia in rats. In ischemic paw edema, iv administration of either NP or NPH4 more potently inhibited the increase of paw thickness after release from ischemia than did administration of superoxide dismutase plus catalase or allopurinol. In gastric ischemia, NP and NPH4 also significantly suppressed the formation of gastric mucosal erosions. Lipid peroxidation in the stomach was increased by ischemia-reperfusion treatment, and the increase was inhibited by the administration of NP or NPH4. The minimum dose of NPH4 required to suppress the gastric ischemic injury in this experiment was 0.3 mg/kg of body weight. These results suggest that neopterin may be effective as a protective agent against ischemia-reperfusion injury, in which active oxygen species are believed to play a major role.

Topics: Allopurinol; Animals; Biopterins; Catalase; Disease Models, Animal; Dose-Response Relationship, Drug; Edema; Gastric Mucosa; Lipid Peroxidation; Male; Mice; Neopterin; Rats; Rats, Sprague-Dawley; Reactive Oxygen Species; Reperfusion Injury; Superoxide Dismutase