OxyFile #235

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

Author: Nishioka T; Kimura M; Takahama U

Source: Arch Oral Biol 1996 Oct; 41(10):911-7


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 HOSC