Role of nitric oxide in the synthesis of guanidinosuccinic acid, an activator of the N-methyl-D-aspartate receptor

Background. We propose that reactive oxygen and argininosuccinic acid (ASA) form guanidinosuccinic acid (GSA). An alternative to this hypothesis is the so-called guanidine cycle, which consists of a series of hydroxyurea derivatives that serve as intermediates in a pathway leading from urea to GSA. We compare the role of the guanidine cycle to that of nitric oxide (NO) in the synthesis of GSA. Methods, The members of the guanidine cycle (hydroxyurea, hydroxylamine plus homoserine, L-canaline, and L-cana-vanine) were incubated with isolated rat hepatocytes. The known NO donors, NOR-2, NOC-7, and SIN-1, were incubated with ASA in vitro. Ornithine, arginine, or citrulline, which increase arginine, a precursor of NO, were incubated with isolated rat hepatocytes. GSA was determined by high-performance liquid chromatography. Results. None of guanidine cycle members except for urea formed GSA. SIN-1, which generates superoxide and NO formed GSA, but other simple NO donors, did not. Both carboxy-PTIO, a scavenger of NO, and dimethyl sulfoxide, a hydroxyl radical scavenger, completely inhibited GSA synthesis by SIN-1. GSA formation by SIN-1 reached a maximum at 0.5 mmol/L and decreased at higher concentrations. GSA synthesis, stimulated by urea in isolated hepatocytes, was inhibited by ornithine, arginine, or citrulline with ammonia, but not by ornithine without ammonia, where arginine production is limited. Conclusion. GSA is formed from ASA and the hydroxyl radical. When arginine increased in hepatocytes, OSA synthesis decreased. These data suggest that increased NO, which results from high concentrations of arginine, or SIN-I scavenges the hydroxyl radical. This may explain the decreased GSA synthesis in inborn errors of the urea cycle where ASA is decreased, and also the diminished GSA excretion in arginemia.