Citrulline

Citrulline is an amino acid whose side chain has the structure $-(\mathrm{C}{\mathrm{H}}_2{)}_3-\mathrm{NH}-\mathrm{C}(=\mathrm{O})-\mathrm{N}{\mathrm{H}}_2$. Its free, uncharged molecular formula is ${\mathrm{C}}_6{\mathrm{H}}_{13}{\mathrm{N}}_3{\mathrm{O}}_3$; the biological L form has a specified arrangement at its chiral carbon atom. Like other amino acids, its amino group and carboxyl group can change protonation state, so a structural formula and the dominant ionic form in a solution need not look identical.^[ref]

An illuminating experimental role is as a reaction product of nitric oxide synthase. A 1990 purification from rat cerebellum associated conversion of arginine to nitric oxide with stoichiometric production of citrulline. The reaction required NADPH and calcium ions, and the purified enzyme required calmodulin. Measuring newly formed citrulline therefore provided a chemically distinct readout of the same enzyme reaction that generated a gaseous signalling molecule.^[ref]

For an idealized assay in which each turnover gives one molecule of citrulline and no competing reaction consumes it, formation of $10\mathrm{nmol}$ of citrulline would correspond to $10\mathrm{nmol}$ of nitric oxide generated. That inference requires the specified reaction and appropriate controls; an existing pool of citrulline in a cell is not by itself a measure of current nitric oxide synthase activity.