Illustration of the tracers used on days 1 and 3, and their metabolic fates. (a) Day 1: [1-¹³C]leucine (I) is converted to [1-¹³C]α-ketoisocaproate (II), which liberates ¹³CO₂ (III) during its oxidative decarboxylation. L[guanidino-¹⁵N₂]arginine (IV) liberates ¹⁵NO (V) and L-[¹⁵N-ureido]citrulline (VI) through the activity of NO synthase. (b) Day 3: L-[¹³C]ureido[5,5,H₂]citrulline (VII) is converted to L-[¹³C-guanidino, 5,5,²H₂] argenine (VIII), which is used as a measure of de novo arginine synthesis. L-[5-¹³C]arginine (IX) is converted to L-[5-¹³C]ornithine (X), which is then converted to L-[5-¹³C]glutamic acid (XI). The latter, via transamination, is then converted to [5-¹³C]α-ketoglutarate (XII), which upon decarboxylation by α-ketoglutarate dehydrogenase yields [¹³C]succinyl CoA (not shown). This releases its ¹³C as ¹³CO₂ (III) during subsequent revolutions of the citric acid cycle.