Antioxidant activity of rat parotid saliva

Background: The healing-promotion property of saliva has been observed in the past, but its underlying mechanism has never been elucidated. We hypothesized a mechanism based on salivary proteins binding to redox active metal ions, rendering them nonactive in their capacity for free radical production. Methods: Examination of this mechanism was conducted by comparing the redox activity of protein-rich saliva with protein-poor saliva. We also examined the redox activity mediated by these 2 kinds of saliva following the in vitro addition of iron, copper, and manganese. Saliva samples were analyzed for their redox activity by measuring the ascorbate-driven and saliva (diluted 1:2)-mediated conversion of salicylate to its 2,3- and 2,5- dihydroxybenzoates and catechol metabolites. Results: The concentrations of salicylate metabolites formed by protein-rich saliva were significantly lower by 45% (P<.05), 66% (P<.01), and 54% (P<.05), respectively, when compared with those formed by protein-poor saliva. The capacity of saliva in suppressing redox activity was found to be inversely related to the concentrations of iron and copper added (but not manganese), but correlated well with the protein content. When the highest concentrations of iron (15 μmol/L) and copper (10 2mmol/L) were added to protein-rich saliva, the concentrations of salicylate metabolites produced were only 0.3% to 1% of those of non-saliva-containing controls (P<.01). However, when these concentrations of iron and copper were added to protein-poor saliva, significantly higher values of redox activity were detected, and the concentrations of the salicylate derivatives produced were 2.1% to 8.1% of those of non-saliva-containing controls (P<.01). In contrast, when the lowest concentrations of iron (2 μmol/L) and copper (0.1 μmol/L) were added, 2.8 to 4 times lower concentrations of salicylate derivatives were produced (P<.01). Conclusion: These results substantiate our hypothesis that saliva has a profound capacity for reducing redox activity rendered by transition metal ions, correlating well with its protein content.