Effects of pH and thiocyanate on hydrogen peroxide-induced evolution of molecular oxygen in human mixed saliva

Hydrogen peroxide-induced evolution of molecular oxygen was measured with a Clark-type electrode in a buffered reaction mixture containing mixed whole or dialysed saliva. The optimum pH for oxygen evolution in mixed whole saliva was around 8. Oxygen evolution was also observed in dialysed saliva, suggesting that free SCN- is not essential. The optimum pH was around pH 6. Sodium thiocyanate inhibited the oxygen evolution under acidic conditions in dialysed saliva, increasing the K(m) of hydrogen peroxide and decreasing the Vmax. Ferric chloride (1 mM), a chelator of SCN-, also inhibited oxygen evolution in dialysed saliva; activity was completely restored by 10 mM sodium citrate. Under alkaline conditions, NaSCN slightly enhanced the oxygen evolution without affecting the K(m) of hydrogen peroxide but increasing the Vmax. Hydrogen peroxide-induced oxidation of SCN- in dialysed saliva was much faster at pH 5 than pH 8. These findings suggest that a function of peroxidase in stimulated saliva where the pH is typically between 7 and 8 is the scavenging of hydrogen peroxide to produce molecular oxygen but without producing OSCN- plus HOSCN.