| Pathway/Step | Enzyme(s) | Substrate(s) | Product(s) | Biological significance |
|---|---|---|---|---|
| ARG1 core reaction | ARG1 (arginase-1) | L-arginine + H2O | L-ornithine + urea | Final cytosolic step of the hepatic urea cycle; detoxifies ammonia by enabling urea excretion and generates ornithine as a branch-point metabolite for downstream biosynthesis (pqac-00000002, pqac-00000006, pqac-00000010) |
| Polyamine pathway: ornithine to putrescine | ODC/ODC1 (ornithine decarboxylase) | L-ornithine | Putrescine | First rate-limiting step in polyamine synthesis; supports cell proliferation, repair, and metabolic reprogramming in immune and tumor contexts (pqac-00000053, pqac-00000055, pqac-00000058) |
| Polyamine pathway: putrescine to spermidine | SRM (spermidine synthase) | Putrescine | Spermidine | Builds polyamine pools that support proliferation, differentiation, mitochondrial fitness, and epigenetic regulation (pqac-00000054) |
| Polyamine pathway: spermidine to spermine | SMS (spermine synthase) | Spermidine | Spermine | Extends polyamine pathway to support cell growth, migration, differentiation, and broader anabolic programs (pqac-00000054) |
| Proline/collagen pathway: ornithine transamination | OAT (ornithine aminotransferase) | L-ornithine | L-glutamate 5-semialdehyde / P5C precursor | Directs ARG1-derived ornithine toward proline biosynthesis, linking arginine catabolism to tissue repair, extracellular matrix production, and fibrosis/collagen programs (pqac-00000053, pqac-00000055, pqac-00000056) |
| Proline/collagen pathway: P5C to proline | PYCR | P5C (1-pyrroline-5-carboxylate) | Proline | Supplies proline for collagen synthesis and extracellular matrix deposition, especially in wound healing, tissue remodeling, and reparative macrophage programs (pqac-00000054) |
| Competing arginine-utilization pathway | NOS (nitric oxide synthase; iNOS/eNOS/nNOS) | L-arginine | Nitric oxide (NO) + citrulline | Competes directly with ARG1 for L-arginine. NOS supports antimicrobial defense, vasodilation, and signaling, whereas ARG1 diverts arginine toward ornithine/polyamine/proline production. Although NOS has higher substrate affinity, arginase has a 10^3-10^4 higher Vmax, making ARG1 a powerful competitor for the shared substrate pool (pqac-00000053, pqac-00000054, pqac-00000055, pqac-00000059) |


*Table: This table summarizes the main biochemical fates of ARG1-generated ornithine and the competing NOS branch from arginine. It is useful for understanding how ARG1 links the urea cycle to polyamine synthesis, proline/collagen production, and nitric-oxide-related signaling.*