NOS2 and chronic obstructive pulmonary disease: Thus, we quantified the l-arginine metabolites l-arginine, l-ornithine and l-citrulline, as well as surrogate markers of NO synthase activity (FeNO and NOx) and dysfunction (the endogenous competitive NOS inhibitor ADMA) in sputum samples obtained from patients with COPD and examined their relationship(s) with the outcomes of lung function testing.