Effect of 5,6,7,8-tetrahydroneopterin on the bovine endothelial cell injury induced by cumene hydroperoxide

Mass spectroscopy and molecular modeling predict endothelial nitric oxide synthase dimer collapse by hydrogen peroxide through zinc tetrathiolate metal-binding site disruption

Endothelial nitric oxide synthase (eNOS) is inhibited by hydrogen peroxide (H(2)O(2)), but the mechanism has not been determined. Thus, the purpose of this study was to delineate the mechanism by which H(2)O(2) inhibits eNOS activity. Using mass spectroscopy, we found that the tetrathiolate cysteine residues 94 and 99 were susceptible to oxidation by H(2)O(2). Molecular modeling predicted that these cysteic acid modifications would disrupt the van der Waals interactions and the hydrogen bonding network mediated by the tetrathiolate cysteines 94 and 99 resulting in changes in quaternary structure, zinc release, and dimer collapse. Using recombinant human eNOS (heNOS) to test the predictions of the molecular modeling we found that H(2)O(2) caused disruption of the heNOS dimer and this was accompanied by zinc release and decreased NO generation. We also found that H(2)O(2) increased the oxidation of tetrahydrobiopterin (BH(4)) to dihydrobiopterin (BH(2)), whereas preincubation of heNOS with excess BH(4) prevented the destruction of zinc tetrathiolate and dimer collapse and preserved activity. Interestingly, we found that the dimmer-stabilizing effect of BH(4) is due to its ability to act as a catalase mimetic. Further, we confirmed that, in ovine aortic endothelial cells, H(2)O(2) could also induce dimer collapse and that increasing cellular BH(4) levels could maintain eNOS in its dimeric form and NO signaling when cells were challenged with H(2)O(2). This study links the inhibitory action of H(2)O(2) on heNOS through the destruction of zinc tetrathiolate metal-binding site and dimer collapse both in vitro and in vivo.

Hydrogen peroxide stimulates tetrahydrobiopterin synthesis through activation of the Jak2 tyrosine kinase pathway in vascular endothelial cells

Tetrahydrobiopterin (BH4) is an essential cofactor for nitric oxide synthase (NOS). We previously described that hydrogen peroxide (H(2)O(2)) increases BH4 levels through the induction of GTP-cyclohydrolase I (GTPCH), which is the rate-limiting enzyme for the synthesis of BH4, in vascular endothelial cells. The aim of this study was to examine the underlying mechanism of H(2)O(2)-induced BH4 synthesis in vascular endothelial cells. The increases in BH4 levels induced by H(2)O(2) were strongly reduced by a Janus kinase-2 (Jak2) inhibitor, AG490. The H(2)O(2)-induced increases in GTPCH mRNA expression and GTPCH activity were also blocked by treatment with AG490. H(2)O(2) elicited an increase in the level of phosphorylated Jak2, suggesting that the induction of BH4 by H(2)O(2) was mediated by the Jak2 pathway. Signal transducers and activators of transcription (Stats) are the best-known substrates for Jak2. The H(2)O(2)-induecd increases in BH4 levels were reduced by treatment with fludarabine, which is shown to cause a specific depletion of Stat1 protein but not of other Stats. Moreover, H(2)O(2) caused the DNA binding of Stat1, and this was inhibited by AG490. Stat1 phosphorylation was enhanced by H(2)O(2) treatment, and the phosphorylation was attenuated by AG490. These findings suggest that the stimulation of BH4 synthesis through the induction of GTPCH is mediated at least in-part by the Jak2-Stat1 pathway.

(6R)-5,6,7,8-tetrahydro-L-biopterin and its stereoisomer prevent ischemia reperfusion injury in human forearm

OBJECTIVE

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.

METHODS AND RESULTS

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.

CONCLUSION

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.

Hydrogen peroxide stimulates tetrahydrobiopterin synthesis through the induction of GTP-cyclohydrolase I and increases nitric oxide synthase activity in vascular endothelial cells

Tetrahydrobiopterin (BH4), which is an essential cofactor for nitric oxide synthase (NOS), is generally accepted as an important molecular target for oxidative stress. This study examined whether hydrogen peroxide (H(2)O(2)), one of the reactive oxygen species (ROS), affects the BH4 level in vascular endothelial cells (ECs). Interestingly, the addition of H(2)O(2) to ECs markedly increased the BH4 level, but not its oxidized forms. The H(2)O(2)-induced increase in the BH4 level was blocked by the inhibitor of GTP-cyclohydrolase I (GTPCH), which is the rate-limiting enzyme of BH4 synthesis. Moreover, H(2)O(2) induced the expression of GTPCH mRNA, and the inhibitors of protein synthesis blocked the H(2)O(2)-induced increase in the BH4 level. The expression of the inducible isoform of NOS (iNOS) was slightly induced by the treatment with H(2)O(2). Additionally, the L-citrulline formation from L-arginine, which is the marker for NO synthesis, was stimulated by the treatment with H(2)O(2), and the H(2)O(2)-induced L-citrulline formation was strongly attenuated by NOS or GTPCH inhibitor. These results suggest that H(2)O(2) induces BH4 synthesis via the induction of GTPCH, and the increased BH4 is coupled with NO production by coinduced iNOS. H(2)O(2) appears to be one of the important signaling molecules to regulate the BH4-NOS system.

Reduction by tetrahydrobiopterin of H2O2-induced endothelial cell injury

The purpose of this study was to examine the effect of tetrahydrobiopterin, a co-factor of nitric oxide synthase, on H2O2-induced endothelial cell injury. Pretreatment with sepiapterin, a precursor of tetrahydrobiopterin biosynthesis, increased tetrahydrobiopterin content of endothelial cells, and reduced H2O2-induced endothelial cell injury, which was measured by leakage of lactate dehydrogenase. Both the increase in tetrahydrobiopterin content and the protective effect of sepiapterin were prevented by co-pretreatment with N-acetylserotonin, an inhibitor of sepiapterin reductase. Although Ca2+ ionophore ionomycin-induced nitric oxide synthesis was increased by pretreatment with sepiapterin, the protective effect of sepiapterin was not affected by an inhibitor of nitric oxide synthesis. On the other hand, pretreatment with sepiapterin also reduced H2O2-induced rat foetal lung fibroblast cell injury via an increase in tetrahydrobiopterin content, despite rat foetal lung fibroblast cells lacking nitric oxide synthase. Moreover, increase in tetrahydrobiopterin strongly reduced H2O2-induced intracellular oxidative stress. These findings indicate that sepiapterin reduces H2O2-induced endothelial cell injury via an increase in tetrahydrobiopterin content. Although increase in endothelial tetrahydrobiopterin content stimulated nitric oxide production, the protective effect of tetrahydrobiopterin against H2O2-induced endothelial cell injury is unlikely to be related to the stimulation of nitric oxide release from nitric oxide synthase. The protective effect of tetrahydrobiopterin may involve reactive oxygen species-scavenging activity.

Total plasma antioxidant capacity is not always decreased in sepsis

OBJECTIVE

To compare total plasma antioxidant capacity and selected individual antioxidants in patients with varying degrees of severity of sepsis.

DESIGN

A prospective, observational, consecutive case study.

SETTING

A 16-bed intensive care unit (ICU) in a university teaching hospital.

INTERVENTIONS

None.

PATIENTS

Forty-six healthy controls, ten ICU patients, nine patients with systemic inflammatory response syndrome (SIRS), 11 septic patients, and 14 septic shock patients. Plasma was obtained in healthy patients scheduled for minor surgery immediately before anesthesia and in ICU patients within 24 hrs of admittance to the unit or diagnosis of SIRS, sepsis, or septic shock.

MEASUREMENTS AND MAIN RESULTS

Using the total peroxyl radical trapping method, we found plasma antioxidant capacity to be lower in septic patients but higher in septic shock patients, as compared with controls. Bilirubin was the greatest contributor to the increase with shock, followed by uric acid. Neopterin also correlated with the peroxyl radical trapping antioxidant parameter values.

CONCLUSION

Although total plasma antioxidant capacity is decreased from normal levels in septic patients, an increase in some oxidants contributes to an increased total antioxidant capacity in septic shock patients.

Effect of 5,6,7,8-tetrahydroneopterin on oxidative modification of low-density lipoprotein, and its uptake in the macrophage-like cell line J774

Since oxidative modification of low-density lipoprotein (LDL) and uptake of the oxidized LDL by macrophages are closely related to the pathogenesis of atherosclerosis, we examined the inhibitory effect of 5,6,7,8-tetrahydroneopterin (NPH4) on Cu(II)-induced lipid peroxidation of LDL. NPH4 significantly inhibited both lipid peroxidation and the associated increase of electrophoretic mobility of ApoB protein. Furthermore, NPH4 suppressed the uptake of oxidized [14C]oleyl-esterified LDL by the macrophage-like cell line J774. NPH4 may be a candidate for the treatment of atherosclerosis and other active oxygen-related diseases.