Figure 1

The reactions of the Krebs-Henseleit (urea) cycle, and their relations to NOS activity. Enzymes: 1. ornithine carbamoyl transferase, 2. argininosuccinate synthetase, 3. argininosuccinate lyase, 4. arginase, 5. nitric oxide synthase, 6. arginine deiminase, 7. arginine decarboxylase, 8. numerous enzymes contributing to the formation of substituted guanidino compounds. Reactions 1 to 4 comprise the urea cycle. Reactions 2, 3, 5, may account for NOS activity in plants (citrulline-NO cycle). Arginine deiminase was reported in chloroplasts but is mostly found in microorganisms. Reactions 7 and 8 comprise decarboxylation, oxidation, methylation, transamidination, phosphorylation, keeping the guanidino group intact or modifying it by methylation, phosphorylation, etc. They remove L-arginine as a substrate from the urea cycle, and from reactions 5 and 6. L-arginine represents an important branch point that links nitrate and ammonium nutrition to protein synthesis and turnover (not shown), to the urea cycle, to a postulated citrulline-NO cycle, and to the formation of guanidino compounds (substituted and non substituted). Through oxygen requirements, the stress-induced NOS activity links respiration to NO, ROS and RNS production, their signaling pathways and damaging reactions, e.g., the nitration of phenols and tyrosines residues in cell regulatory proteins, and to apoptosis.