Immunocytochemical localization of xanthine oxidase in rat myocardium

Monoclonal antibodies (MAb's) to xanthine oxidase (XO) (from bovine milk) were produced by hybridoma technique. Culture supernatants were initially screened using enzyme-linked immunoabsorbant assay (ELISA). Forty four positive clones were subcloned and further characterized by ELISA and immunoblot analysis. Out of fifteen clones, which were positive in immunoblot analysis, one clone N2-26 was also positive in immunocytochemical studies. Indirect immunoperoxidase and enzyme histochemistry staining showed that XO activity is present in endothelial cells of capillaries, small blood vessels and also in interstitial cells. Electron microscopy revealed that diaminobenzidine reaction product was distributed in the cytoplasm of interstitial cells and endothelial cells of capillaries and small blood vessels. This is the first report of the presence of XO in interstitial cells and endothelial cells of small blood vessels. Allopurinol, which inhibits the xanthine oxidase activity, did not have any effect on the immunocytochemical staining. Our results in normal rat heart suggest that XO activity is confined to interstitial cells, endothelial cells of capillaries and small blood vessels.