The
hypothiocyanite ion (
OSCN-) is the principal oxidation product of the salivary
peroxidase-
thiocyanate (SCN-)-
hydrogen peroxide antimicrobial system. Supplementation of human saliva in vitro and in vivo with low amounts (less than 1.0 mM) of
hydrogen peroxide increase the concentration of salivary
OSCN- (in vivo up to 0.3 mM). Elevated concentrations of
OSCN- are strongly antimicrobial and may therefore be protective against
dental caries. However, as
OSCN- is a highly-reactive
oxidizing agent, its possible toxic effect on human cells was studied using gingival fibroblasts as target cells. Concentrations of
OSCN- (up to 300 microM) had no effect on [3H]-
thymidine incorporation into the cells. However, fibroblasts were sensitive to
peroxide so that 100 microM of H2O2 caused over 80 per cent reduction in [3H]-
thymidine incorporation. The toxicity of H2O2 could be entirely prevented by adding
lactoperoxidase and SCN- to the cell culture before the addition of
peroxide. Thus, conversion of toxic H2O2 to non-toxic
OSCN- in fibroblast culture by
lactoperoxidase and SCN- suggests a dual role for the salivary
peroxidase system: protection of human cells from H2O2 toxicity and antimicrobial action against oral pathogens. Furthermore, the elevated concentrations of
OSCN- which produce inhibition of bacterial metabolism did not damage human cells.