(6R)-5,6,7,8-tetrahydro-L-biopterin modulates nitric oxide-associated soluble guanylate cyclase activity in the rat cerebellum

Tetrahydrobiopterin protects against guanabenz-mediated inhibition of neuronal nitric-oxide synthase in vitro and in vivo

It is established that guanabenz inhibits neuronal nitric-oxide (NO) synthase (nNOS) and causes the enhanced proteasomal degradation of nNOS in vivo. Although the time- and NADPH-dependent inhibition of nNOS has been reported in studies where guanabenz was incubated with crude cytosolic preparations of nNOS, the exact mechanism for inhibition is not known. Moreover, even less is known about how the inhibition of nNOS triggers its proteasomal degradation. In the current study, we show, with the use of purified nNOS, that guanabenz treatment leads to the oxidation of tetrahydrobiopterin and formation of a pterin-depleted nNOS, which is not able to form NO. With the use of 14C-labeled guanabenz, we were unable to detect any guanabenz metabolites or guanabenz-nNOS adducts, indicating that reactive intermediates of guanabenz probably do not play a role in the inhibition. Superoxide dismutase, however, prevents the guanabenz-mediated oxidation of tetrahydrobiopterin and inhibition of nNOS, suggesting the role of superoxide as an intermediate. Studies in rats show that administration of tetrahydrobiopterin prevents the inhibition and loss of penile nNOS due to guanabenz, indicating that the loss of tetrahydrobiopterin plays a major role in the effects of guanabenz in vivo. Our findings are consistent with the destabilization and enhanced degradation of nNOS found after tetrahydrobiopterin depletion. These studies suggest that drug-mediated destabilization and subsequent enhanced degradation of protein targets will likely be an important toxicological consideration.

Effects of tetrahydrobiopterin on ataxia in Machado-Joseph disease may be based upon the theory of 'cerebellar long-term depression'

We previously verified the effectiveness of tetrahydrobiopterin (BH4) on the ataxia in Machado-Joseph disease (MJD; SCA3 [spinocerebellar ataxia type 3]) as one of the most common types of dominantly inherited spinocerebellar ataxias. We hypothesized as to the pharmacological mechanism of BH4 that on the basis of 'cerebellar long-term depression' theory, BH4 may exert its actions at the levels of soluble guanylate cyclase in the Purkinje cells and of nitric oxide synthase in the granule cells. If cerebellar long-term depression is the case as the theoretical basis of BH4, it will open a new page of therapeutic strategy for spinocerebellar ataxias.

Acute effects of tetrahydrobiopterin on the dynamic characteristics and adaptability of of vestibulo-ocular reflex in normal and flocculus lesioned rabbits

The acute effects of (6R)-5,6,7,8-tetrahydro-L-biopterin (R-THBP) on the dynamic characteristics of horizontal vestibulo-ocular reflex (HVOR) were examined in chronically prepared alert pigmented rabbits. The HVOR gain was measured by 10 degrees-0.1 Hz sinusoidal oscillation of the turntable in darkness. In control rabbits, intramuscular injection of R-THBP (10 mg/kg) induced an increase of HVOR gain by 0.1-0.2, which lasted for 1-3 h. Sustained 10 degrees-0.1 Hz sinusoidal oscillation of the turntable and screen in the reversed direction induced an adaptive increase of HVOR gain by 0.2 in 3 h, which was not affected by applications of R-THBP. In flocculus lesioned rabbits, the HVOR gain reduced by 23%, and applications of R-THBP no longer induced a HVOR gain increase. These experimental observations suggested that the R-THBP affected on the HVOR dynamics within the cerebellar flocculus or its related neural areas.

Acute amelioration of diabetic endothelial dysfunction with a derivative of the nitric oxide synthase cofactor, tetrahydrobiopterin

Tetrahydrobiopterin is a cofactor for nitric oxide synthase. In low concentrations of this cofactor, nitric oxide synthase is known to produce less nitric oxide and, correspondingly, enhanced quantities of the oxidant species, hydrogen peroxide. In this study, we tested the hypothesis that an exogenous tetrahydrobiopterin derivative might improve endothelial nitric oxide synthase activity in diabetic endothelium. Diabetes was induced in Sprague-Dawley rats with intravenous injections of streptozotocin. After 8 weeks, endothelium-dependent relaxation was assessed in aortic rings by using acetylcholine, whereas endothelium-independent relaxation was assessed by using nitroglycerin. Acetylcholine-induced relaxation was impaired in diabetic rings, whereas nitroglycerin-induced relaxation was unimpaired. Exposure of rings for 30 min with 100 microM of the pteridine derivative, 6-methyl-5,6,7,8-tetrahydropterin (in the presence of diethylenetriaminepentaacetic acid to inhibit oxidation), followed by washing and equilibration in control media, augmented relaxation induced by acetylcholine in diabetic rings but had no effect on relaxation in control rings. Pteridine exposure did not alter relaxation or sensitivity to nitroglycerin in control rings either with or without endothelium. In diabetic rings, pteridine exposure augmented maximal relaxation to nitroglycerin in rings with or without endothelium while increasing the sensitivity only in rings with endothelium but not in rings without endothelium. In contrast, there was no effect of pteridine exposure on relaxation or sensitivity to nitroglycerin in diabetic rings (with or without endothelium) that are pretreated with L-nitroarginine. In summary, tetrahydrobiopterin availability can play a key role in the regulation of nitric oxide production by diabetic endothelium.

TPA and cycloheximide modulate the activation of NF-kappa B and the induction and stability of nitric oxide synthase transcript in primary neonatal rat hepatocytes

12-O-Tetradecanoylphorbol 13-acetate (TPA) elicited a transient increase in the transcription of the inducible nitric oxide synthase (iNOS) gene coupled with a shortening of the half-life of its mRNA in primary neonatal rat hepatocytes. These effects of TPA were preceded by a surge in nuclear translocation of the transcription factor NF-kappa B, and followed by a mounting accumulation of NO-2 in the growth medium. Even cycloheximide (CHX) added by itself elicited an early, sustained activation of NF-kappa B followed by an intense induction of iNOS gene expression, irrespective of what degree of protein synthesis inhibition was brought about by the several concentrations tested. When given together, TPA and CHX exerted additive effects on hepatocellular iNOS mRNA levels. These results suggest the likelihood of an ordered sequence of events by which an activated NF-kappa B mediates the induction of iNOS gene expression in TPA- and/or CHX-treated primary hepatocytes.

Sulfamethoxazole-trimethoprim double-blind, placebo-controlled, crossover trial in Machado-Joseph disease: sulfamethoxazole-trimethoprim increases cerebrospinal fluid level of biopterin

We performed a double-blind, placebo-controlled, crossover trial of sulfamethoxazole-trimethoprim (S-T) in 8 patients with Machado-Joseph disease (MJD), and measured the blood and cerebrospinal fluid levels of biopterins, biogenic amines or metabolites, and folate. The clinical results were as follows; mild improvements of hyperreflexia of knee jerks and of rigospasticity of the legs during S-T treatment period. In addition, S-T significantly reduced the times of 8 motor activities on the timed tests. The biochemical results showed that basal levels of all biopterins and homovanillic acid in the cerebrospinal fluid (CSF) were reduced to less than half the levels of those of controls with other neurological diseases. After S-T treatment, total and oxidized form of biopterins in the CSF increased significantly. Therefore, S-T may be effective to neurologic deficits through its mechanism of increasing the level of brain biopterins.