Zinc Inhibition of the Generation of Indole/Skatole during Oral Malodor Formation

Introduction and objective: Zinc reduction of volatile sulfur compound (VSC) generation during oral malodor formation is well established. But, zinc effects on indole/skatole have hardly been studied. Indole and skatole (IS) arise from bacterial degradation of tryptophan (trp) and can contribute significantly to the oral malodor "bouquet". Object here was to study zinc inhibition of IS production. Methods: Stimulated whole saliva from 12 h fasting subjects was centrifuged at 1740 g to produce bacteria containing salivary sediment. Sediment at 8.3% (v/v) was mixed with cysteine or trp at 6 mM or salivary supernatant (SS) at 33.3% (v/v), and incubated at 37^oC for 24 h. ZnCl₂ at 6 mM was added to each and head-space odor was monitored (i) organoleptically and (ii) for VSC with the Halimeter at 0, 15 and 30 min and at 1, 2, 3, 4, 8 and 24 h. At 24 h, IS was measured by Kovac's method. Results: Sediment incubated with trp produced 13.8 ±1.27(SEM) mg IS /ml and 7.45 ±0.07(SEM) mg IS /ml with supernatant. Zinc did not affect IS production from trp, but did (at 48%) with supernatant. Organoleptically, zinc showed 42% malodor reduction with trp and 43% malodor reduction with supernatant. Zinc reduced VSC production from cysteine by 70% and VSC from supernatant by 49%. Because zinc did not affect IS formation from trp, organoleptic odor inhibition with zinc present could not be due to IS production. Conclusion: As explanation, supernatant has both free and protein bound trp. Bound trp is released by bacterial proteolytic activity. Zinc inhibition of IS generation from supernatant may therefore be related to its effects on trp releasing proteolytic activity. Besides anti-VSC properties, zinc may also be able to indirectly control indolic compound contribution to the breath.