Background & aims: Inducible nitric oxide synthase (iNOS) is generated in several cell types by treatment with lipopolysaccharides or cytokines. Earlier studies suggested that ulcerative colitis is associated with increased NO produced by iNOS; however, the cellular source of the NO synthesis was not identified. A possible mechanism of NO-induced cellular damage is through its interaction with superoxide to produce peroxynitrite, which reacts with tyrosine to form nitrotyrosine in cellular proteins.
Methods: Using immunoperoxidase microscopy with a new monospecific human iNOS antibody (NO-53), the cellular distribution of iNOS and nitrotyrosine was examined using human colonic mucosa from normal bowel, ulcerative colitis, Crohn's disease, and diverticulitis.
Results: Intense focal iNOS labeling was localized to the inflamed colonic epithelium in ulcerative colitis, Crohn's disease, and diverticulitis but was not detectable in the uninflamed epithelium. Nitrotyrosine labeling was also observed in the inflamed colonic epithelium and was associated with nearby iNOS staining; nitrotyrosine was undetectable in normal mucosal epithelium. iNOS and nitrotyrosine were also detected in lamina propria mononuclear cells and neutrophils.
Conclusions: These findings suggest that iNOS is induced in the inflamed human colonic epithelium and is associated with the formation of peroxynitrite and the nitration of cellular proteins.