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 w ith a Clark-type electrode in a buffered reaction mixture containing m ixed 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 opti mum pH was around pH 6. Sodium thiocyanate inhibited the oxygen evolut ion under acidic conditions in dialysed saliva, increasing the K-m of hydrogen peroxide and decreasing the V-max. 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 alkali ne conditions, NaSCN slightly enhanced the oxygen evolution without af fecting the K-m of hydrogen peroxide but increasing the V-max. Hydroge n peroxide-induced oxidation of SCN- in dialysed saliva was much faste r at pH 5 than pH 8. These findings suggest that a function of peroxid ase in stimulated saliva where the pH is typically between 7 and 8 is the scavenging of hydrogen peroxide to produce molecular oxygen but wi thout producing OSCN- plus HOSCN. Copyright (C) 1997 Elsevier Science Ltd.