2,4-DIAMINO-6-HYDROXYPYRIMIDINE, AN INHIBITOR OF GTP CYCLOHYDROLASE-I, SUPPRESSES NITRIC-OXIDE PRODUCTION BY CHICKEN MACROPHAGES

Biosynthesis of nitric oxide (.NO) from L-arginine by nitric oxide syn thase (NOS) represents a major cytotoxic effector function of macropha ges. It has been shown that most mammalian NOS requires tetrahydrobiop terin (BH4) as a cofactor and that inhibition of BH4 synthesis results in suppressed .NO production. Chicken L-arginine metabolism differs f rom that of mammals in that chickens cannot synthesize L-arginine de n ovo. Therefore, it is important to examine whether chicken macrophage .NO synthesis is also BH4-dependent. 2,4-diamino-6-hydroxypyrimidine ( DAHP), a specific inhibitor for GTP cyclohydrolase I (GTP-CH; EC 3.5.4 .16), the rate-limiting enzyme in de novo pterin synthesis, was used t o block synthesis of BH4. Both chicken peritoneal macrophages (PECs) a nd the avian MC29 virus-transformed macrophage cell line, HD11, exhibi ted a dose-dependent reduction in .NO production (measured as nitrite accumulation) relative to DAHP concentration. Authentic BH4 and a subs trate for pterin salvage pathway of BH4 synthesis, sepiapterin, were b oth capable of restoring the production of .NO in DAHP-treated PECs an d HD11 macrophages. These results suggest that chicken macrophages req uire active synthesis of BH4 to produce .NO and that chemicals interfe ring with BH, synthesis may result in suppressed .NO production and, h ence, .NO-mediated immune function.