Nitric oxide inhibits tyrosine hydroxylase of rat median eminence

Abstract Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the biosynthesis of catecholamines. In a previous report we found that intracerebroventricular administration of nitric oxide (NO) generator sodium nitroprusside (SNP) to conscious male rats inhibited dose-dependently the TH activity of the median eminence (ME). In the present study we have tested the in vitro effects of SNP on TH activity, its possible mediator and action mechanism. Exposure of the ME TH to SNP (50, 100 and 500 μM) caused concentration-dependent inhibition of its enzyme activity. Addition of; reduced hemoglobin Hb (10 μM), a NO scavenger, superoxide dismutase SOD (1000 units/ml), a superoxide scavenger enzyme, or uric acid UA (300 μM), a peroxynitrite scavenger, did not affect the enzyme activity by themselves, but prevented the inhibitory effect of SNP 500 μM. However, the presence of methylene blue MB (100 μM), a guanylyl cyclase inhibitor, did not alter either basal enzyme activity or the inhibitory action of SNP 500 μM. These results suggest that this action of SNP on TH of the ME would be mediated by peroxynitrite generated by the reaction of NO with superoxide.