Thermospray tandem mass spectrometric analysis of oxygen incorporation into citrulline by nitric oxide synthase

Citrulline is formed as a co-product in the biosynthesis of nitric oxide from L-arginine by the action of either constitutive or inducible nitric oxide synthase which is present in a variety of cells. We have previously shown that the oxygen atom incorporated into both nitric oxide and citrulline derives from molecular oxygen and not water. This paper describes the tandem mass spectrometric analysis of citrulline synthesized by the macrophage cell line J774 in the presence of native or guanidino-labelled arginine and air or isotopically enriched oxygen. The results confirm that oxygen is incorporated into the ureido position of citrulline.