The Roles of Human-Inducible Nitric Oxide Synthase and Xanthine Oxidase in Regulating Chronic Inflammation in Radicular Cysts

Abstract

The immunohistochemical activities of nitric oxide synthase (NOS), xanthine oxidase(XO) and nitrotyrosine(NT) in human radicular cysts were measured. The molecular interactions of nitric oxide (NO) or peroxynitrate(ONOO-) and superoxide(O2- •) generated in the xanthine/XO system were kinetically investigated in vitro to clarify the mechanism of tissue injury. To investigate the interaction between NO • and O2- • generated in radicular cysts, the localization of NOS and XO was evaluated by an immunohistochemical staining method. Furthermore, the presence of ONOO- was immunohistochemically estimated. The molecular interactions of O2- •, NO • and ONOO- were analyzed by using the electron spin resonance spin-trapping method. In an area of inflammatory reaction, human-inducible NOS (iNOS), XO and NT were positive compared with a normal area, although only iNOS production was confirmed an area of fibrous reaction. Kinetic studies showed that 1-hydroxy-2-oxo-3 (N-methyl-3-aminopropyl)-3-methyl-1-triazene (NOC-7) reacted directly with O2- •, whereas ONOO- did not react with O2- • but inhibited XO directly. The close involvement of iNOS production in the inflammatory region and the influence of the inflammatory reaction with both NO • production from iNOS and O2- • production from XO were suggested. The presence of iNOS in cysts suggests that NO • produced in periapical lesions plays an important role in the regulation of chronic lesions via the reaction with O2- • produced catalyzed by XO. From the reaction with O2- •, and NO • generated by the iNOS system, the formation of ONOO- and its direct inhibition of XO activity decreased the amount of O2- • produced and as a result inhibited the continued formation of ONOO-.