# Decomposition of Flavonols in the Presence of Saliva
## 1 Introduction  
Flavonoids are phenolic compounds widely present in plants and food of plant origin. Both clinical and epidemiological studies show the correlation between the dietary polyphenols intake and the reduction of risk of some chronic diseases such as cardiovascular diseases, cancer, and diabetes, as well as aging [1; 2; 3; 4]. These beneficial effects are associated with their antioxidant activity [5; 6; 7]. Flavonols, the major class of flavonoids present in the human diet, and among them quercetin, kaempfer, myricetin, and isorhamnetin (Figure 1), are well known to act as antioxidants in vitro and show protective effects against free radicals, reactive oxygen species, and other oxidation agents [8; 9; 10]. However, the biological properties of antioxidants such as flavonoids depend on their bioavailability and metabolism in the human body. The study of biological responses due to dietary intake of polyphenols cannot be carried out without taking into consideration polyphenols-saliva interactions. Although phenolics are normally ingested through the mouth as elements of food, very little is known about their metabolism in the oral cavity. One of the main goals assigned to saliva is participation in glycosides hydrolysis [11; 12], which delivers biologically active aglycones that can be absorbed more effectively in the human digestive tract. Since flavonoids mainly occur in food as glycosides, much of the research focused on the metabolism in oral cavity concerns glycosides. However, some reports show the presence of flavonoids aglycones in the food of plant origin, for instance in eucalyptus and uniforal types of honey [13]. Moreover, the fermentation carried out during the manufacturing of food couldresults in the hydrolysis of glycosides to aglycones. Although it was reported that flavonoid glycosides can be absorbed intact via the sodium-dependent glucose transporter [14], it was also shown that many glycosides are not absorbed due to efficient efflux transport by intestinal efflux protein pumps [15]. It is well established that the oral cavity harbors numerous and diverse microorganisms, which can hydrolyze flavonoids glucosides to the aglycones by glucosidases excreted from the bacteria [11; 16]. All of these features indicate that the effective absorption of polyphenols in the human digestive tract strongly depends on their deglycosylation, and, as far as we know, study on the metabolism of aglycones in the oral cavity are limited. Interaction of saliva components with bioactive compounds from food occurs due to various reasons. Firstly, chewing food allows its components to stay in the oral cavity for a while, which ensures the contact of saliva and oral mucosa with food. Secondly, during chewing, flavonoids are dissolved in saliva, which facilitates their interaction with oral mucosa and salivary proteins [17; 18; 19; 20]. It is worth noting that [PERSON] et al. [17] outlined interactions between flavan-3-ols and salivary proteins not only as a precipitation issue as it has been usually studied but also as a more complex interaction, which involves the formation of soluble and insoluble complexes. All of these mechanisms increase lipophilic polyphenols assimilation and causes their retention in the oral cavity over time [21; 22; 23]. The human oral cavity contains numerous and diverse microorganisms as commensals [24; 25]. Approximately 280 bacterial species from the oral cavity have been isolated in culture and formally named. The possible reason for the decomposition of flavonoids in the presence of saliva is the fact that bacteria and leucocytes presented in the oral cavity are able to generate H\({}_{2}\)O\({}_{2}\); thus, flavonoids could be easily oxidized [26; 27]. While there is a lack of reports showing the oxidation of flavonoids in the presence of saliva, other oxidizing conditions have been well established, mainly for quercetin. Quercetin is degraded in air conditions with the formation of different depsides, as intermediates in the degradation pathway.