GUANOSINE TRIPHOSPHATE CYCLOHYDROLASE-I REGULATES NITRIC-OXIDE SYNTHESIS IN RENAL PROXIMAL TUBULES

The synthesis of nitric oxide by proximal tubule-inducible nitric oxid e synthase requires tetrahydrobiopterin as a cofactor. To determine wh ether tetrahydrobiopterin synthesis is required for nitric oxide produ ction, nitrite release by mouse proximal tubule cells treated with 2,4 -diamino-6-hydroxypyrimidine, an inhibitor of the rate-limiting enzyme in the de nova synthesis of tetrahydrobiopterin from guanosine tripho sphate, guanosine triphosphate cyclohydrolase I, was measured. Treatme nt with lipopolysaccharide (0.1 mu g/mL) and interferon-gamma (100 U/m L) for 12 h increased nitrite production from 2.7 +/- 0.2 to 25.4 +/- 1.3 nmol/mg of protein (P < 0.001; N = 9). 2,4-Diamino-6-hydroxypyrimi dine (6 mM) reduced lipopolysaccharide/interferon-gamma-induced nitrit e production by 53.1 +/- 3.4% Sepiapterin, a substrate for tetrahydrob iopterin synthesis via the dihydrofolate reductase-dependent pterin sa lvage pathway, prevented the inhibition by 2,4-diamino-6-hydroxypyrimi dine, an effect that was blocked by methotrexate. In conclusion, guano sine triphosphate cyclohydrolase I activity is required for cytokine-i nduced nitric oxide production by proximal tubular epithelium. The inh ibition of guanosine triphosphate cyclohydrolase I could prove useful in the treatment of nitric oxide-mediated renal disorders.