Increased expression of an inducible isoform of nitric oxide synthase and the formation of peroxynitrite in colonic mucosa of patients with active ulcerative colitis.

Increased production of reactive metabolites of oxygen and nitrogen has been implicated in chronic inflammation of the gut. The object of this study was to examine the magnitude and location of nitric oxide synthase (NOS) activity and peroxynitrite formation in the colonic mucosa of patients with ulcerative colitis in relation to the degree of inflammation. Thirty three patients with active ulcerative colitis (17 with mild or moderate inflammation, 16 with severe inflammation). Inducible NOS activity was determined in the colonic mucosa by measuring the conversion of L-arginine to citrulline in the absence of calcium. The localisation of NOS and nitrotyrosine immunoreactivity was assessed immunohistochemically using the labelled streptavidin biotin method. Inducible NOS activity increased in parallel with the degree of inflammation of the mucosa. Expression of inducible NOS was found not only in the lamina propria, but also in the surface of the epithelium. Peroxynitrite formation as assessed by nitrotyrosine staining was frequently observed in the lamina propria of actively inflamed mucosa. Nitric oxide and peroxynitrite formation may play an important role in causing irreversible cellular injury to the colonic mucosa in patients with active ulcerative colitis.