Tetrahydrobiopterin impairs the action of endothelial nitric oxide via superoxide derived from platelets.

The mechanism by which exogenous tetrahydrobiopterin (BH(4)) impairs the action of endothelial nitric oxide (NO) in the presence of platelets was investigated. The endothelial NO generated by shear stress was determined by the anti-aggregating activity of indomethacin-treated endothelial cells and the cyclic GMP concentration in platelets. The inhibitory effect of exogenous BH(4) was suppressed by superoxide dismutase (SOD), or diclofenac sodium at concentrations inhibiting O(2)(-) generation, but not by allopurinol, a xanthine oxidase inhibitor. BH(4) similarly inhibited the anti-aggregatory effect of sodium nitroprusside (SNP), a NO donor. The inhibitory effect was suppressed by diphenyleneiodonium, a specific inhibitor of NADPH oxidase. Six(S)-BH(4), an inactive diastereoisomer of 6(R)-BH(4), and the 5,6,7,8-tetrahydropterin compounds inhibited the endothelial NO action, whereas sepiapterin and 7,8-dihydrobiopterin (BH(2)), 5,6-double bond pterins, were inactive. These tetrahydropterins, but not sepiapterin and BH(2), scavenged superoxide (O(2)(-)) generated by the hypoxanthine-xanthine oxidase reaction, possibly due to electron transfer during oxidation to its quinonoid-form. BH(4) markedly stimulated the O(2)(-) generation from platelets, in the presence of NADH, rather than that of NADPH. These findings suggest that BH(4) stimulates platelet NAD(P)H oxidase to generate O(2)(-), and inhibits the anti-aggregating effect of NO. SOD activity in the local environment may modify the effect of BH(4) on the endothelial NO activity.