Figure 1.

Arginine metabolism, the partial reactions of the urea cycle, the L-arginine-NO pathway, a citrulline-NO cycle, and a branch point leading to the formation of guanidino compounds with special reference to conifers in eastern Canada. 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 acting on arginine and responsible for the formation of guanidino compounds. Reactions 1 to 4 comprise the partial reactions of the urea cycle in plants. Not shown is the synthesis and turnover of proteins which alters the pool of available arginine and other protein amino acids. Reactions 2, 3, and 5 comprise the L-arginine-NO pathway and the citrulline-NO cycle, which are responsible for the stress-induced formation of NO from arginine and oxygen. 6. Arginine deiminase is not yet known in conifers but has been reported in other plants. Steps 7 and 8 remove arginine from the urea and citrulline-NO cycles and divert N into the naturally occurring guanidino compounds some of which are inhibitors of respiration. During the onset of winter dormancy, arginine N, the guanidino compounds and proline N accumulate in the physiological fluids. Arginine is stored in reserve proteins to provide N for amino acid, amide, and renewed protein and nucleic acid synthesis in the spring. The conversion of proline to glutamine via glutamic acid now provides transferable hydrogen atoms making proline a readily available and highly-water soluble source of energy and reducing power for the photosynthetic assimilation of carbon dioxide.