1 · eNOS produces NO
In the endothelial cells lining vessels, eNOS makes nitric oxide () from L-arginine, O₂ and , with L-citrulline as the byproduct.Embedded in the endothelial cells lining vessel walls is an enzyme — endothelial nitric oxide synthase (eNOS). It runs a 5-electron oxidation on three substrates:L-arginine (Arg) + O₂ + → + L-citrulline + NADP⁺
Note the right side — citrulline shows up again. In this scene it is both the downstream product of the earlier 'oral supplement converted back to Arg' step, and the byproduct of the eNOS reaction. It can be cycled back to Arg by the kidney, forming the small Arg–citrulline cycle.
So the logic 'supplement citrulline → raise plasma Arg → give eNOS a more stable substrate supply' closes biochemically — it is not invented by supplement marketing.
eNOS cofactors: BH₄ (tetrahydrobiopterin), FAD, FMN, heme Fe, CaM (calmodulin) — activated by shear stress, ACh, bradykinin.