pH-dependent fluoride inhibition of peroxidase activity

Fluoride was found to be a potent inhibitor of bovine lactoperoxidase and of salivary peroxidase at acid pH values. Inhibition was reversible at neutral pH, and appeared to involve HF binding by the enzyme. Fluoride inhibition of lactoperoxidase occurred with all reductants tested, including thiocyanate, iodide, and guaiacol. Fluoride concentrations for 50% inhibition of enzymatic activity with iodide as reductant were: less than 0.05 mM at a pH value of 4.0, 0.3 mM at 5.0, 4.0 mM at pH 6.0, and more than 10.0 mM at pH 7.0. Salivary peroxidases were found to have lower pH optima but to be approximately as sensitive to acid-dependent fluoride inhibition as was purified bovine lactoperoxidase. The findings suggest that the fluoride in dental plaque may be inhibitory to the antimicrobial peroxidase system.