Figure 1.

Scheme for arginase catabolism of L-arginine to L-ornithine/urea or L-citrulline/NO, production of polyamines and anabolism and catabolism of proline. Also shown are the pathway for synthesis of L-arginine from L-glutamine, the reversible pathway between L-ornithine and L-glutamine, and the recycling of L-citrulline into L-arginine. Abbreviations: ASL, aminosuccinate lyase; ASS, aminosuccinate synthase; NOS, nitric oxide synthase; OAT; ornithine aminotransferase; ODC, ornithine decarboxylase; OTC, ornithine transcarbamylase. Arginase (bottom, left) is the final enzyme in the urea cycle within the liver, which restarts the cycle through the synthesis of L-citrulline from carbamoyl-phosphate (1) and L-ornithine (2) by OTC (center). It should be noted that that these reactions do not all occur within any given cell. In particular, the urea cycle is independent of the other reactions; i.e., L-arginine produced within the urea cycle is not a substrate for NOS and L-ornithine produced within the urea cycle is not a substrate for OAT or ODC