Citrulline is the L-enantiomer of citrulline. It has a role as an EC 1.14.13.39 (nitric oxide synthase) inhibitor, a protective agent, a nutraceutical, a micronutrient, a human metabolite, an Escherichia coli metabolite, a Saccharomyces cerevisiae metabolite and a mouse metabolite. It is an enantiomer of a D-citrulline. It is a tautomer of a L-citrulline zwitterion.