Abstract:

Saliva has become a central topic of research in many scientific categories. It is involved in mastication, lubrication, buffering action, maintenance of tooth integrity, physicochemical and antimicrobial defense, immunization, wound healing, taste, and early digestion (Amerongen and Veerman, 2002; Fábián et al, 2008). It is also important in biofilm formation on tooth surfaces, bacterial adhesion, serves as an important source for genetic and forensic profiles and maintains mucosal integrity of the oral and upper gastrointestinal mucosal surfaces. The oral cavity can be considered a ‘bio-reactor’ (Gorelik et al, 2008; Kanner et al, 2012) where, on a daily basis, multiple interactions occur among salivary electrolytes, thousands of different proteins including the glycoprotein mucin, plasma-derived albumin, immunoglobulins, digestive enzymes such as alpha amylase as well as with substantial amounts of polyphenols from nutrients. However, saliva also contains potentially toxic H2O2 generated by oral streptococci, salivary lactoperoxidase generates bactericidal and cytocidal thiocyanate anion (SCN−) and activated phagocytes release a series of toxic oxidants (Grisham and Ryan, 1990; Nagler et al, 2002; Halliwell and Gutteridge, 2007). This implies that saliva and the oral structures may be constantly exposed to oxidative stresses. Over the evolution, saliva has evolved the protective low molecular weight antioxidants (LMWA) uric acid, ascorbate, reduced glutathione, and plasma albumin possessing antioxidant activity, is delivered to saliva via the crevicular fluid (Sculley and Langley-Evans, 2002; Liskmann et al, 2007; Ginsburg et al, 2012). However, saliva may also contain red blood cells extravasated either following tooth brushing, use of tooth picks, during orthodontic treatment or in oral pathologies. Erythrocytes had been proposed to serve not only as transporters of oxygen and removers of CO2 but also as ‘sinks’ for reactive oxygen species (ROS) and as protectors of other cells against oxidant toxicity (Richards et al, 1998; Koren et al, 2009, 2010; Ginsburg et al, 2012). It was also proposed that quantifications of antioxidants be performed in whole blood but not exclusively in plasma (Ginsburg et al, 2011b). Today, the main justification to present a ‘letter to the editor’ on salivary functions, stems from a series of recent novel observations, which shed a new insight on the interactions of salivary proteins with polyphenols from nutrients and with blood cells and how such interactions might affect the redox status and the integrity of the oral cavity. The following are the main highlights: Microbial and red blood cells acquired enhanced oxidant-scavenging abilities (OSA) by avidly binding to their surfaces a large assortment of antioxidant polyphenols from nutrients. Such complexes acted in synergy with the antioxidants in whole unstimulated saliva to decompose ROS (Koren et al, 2009, 2010; Ginsburg et al, 2011a). Many of the polyphenols in aqueous beverages (e.g. red wine, tea, coffee, cocoa, cinnamon, cranberries, pomegranate etc.) might not exist in a full soluble state, and therefore not available as effective antioxidants. However, this shortcoming could be overcome by simply mixing the various agents either with fresh un-stimulated saliva (Ginsburg et al, 2012) or with mucin and albumin, which all serve as ‘solubilizers’ of lipophilic agents to render polyphenols more available as efficient antioxidants. Polyphenols in plants and fruit beverages can strongly adhere to the huge surface area of the oral cavity are retained there for long periods and this, despite a normal salivary flow. This suggested that bound polyphenols could act as a ‘slow release apparatus’ helping to maintain a proper redox status and probably also the defense against oxidative stresses (Ginsburg et al, 2012). The OSA in the oral cavity is a sum result of the synergistic interactions among antioxidants in saliva, crevicular fluid, antioxidant polyphenols from nutrients, blood elements, and paradoxically perhaps, also the indigenous catalase-positive colonizing microbial flora (Ginsburg et al, 2011b). Figure 1 represents one example, of many studied, showing a synergistic OSA resulting from the interactions among saliva, whole blood and the tea major polyphenol epigallocatechin gallate (EGCG). However, one has also to take into consideration that the presence in the oral cavity of excessive amounts of heme proteins which occur in periodontal pathologies, may also act as a ‘double-edged sword’ by supplying excessive amounts of Fe+2, instrumental in the generation, via the Fenton reaction, of the highly toxic hydroxyl radical. Antioxidant polyphenols might under certain conditions, also act as pro-oxidants and as signaling molecules which can generate pro-inflammatory agents (Rahman et al, 2006; Halliwell and Gutteridge, 2007; Halliwell, 2008). Figure 1. Luminol-dependent chemiluminescence patterns (Ginsburg et al, 2004) induced by combinations among unstimulated fasting saliva, whole blood, and epigallocatechin gallate (EGCG). The various agents and combinations among them were added to test tubes containing 800 μl of Hanks balanced salt solution (HBSS). This was followed by the addition of a ‘cocktail’ comprised of luminol (10 μM), sodium selenite (1 mM), H2O2 (1 mM) and CoCl2.6H2O (10 μM) which induced a rapid light wave due to H2O2 and hydroxyl radical. Note that sub inhibitory amounts of saliva and of EGCG acted in synergy with whole blood to significantly quench luminescence (n = 5) Because of the very low bioavailability of polyphenols resulting from intensive metabolism in the liver and by the microbial flora of the gastrointestinal tract and due to a strict regulatory mechanism (Ginsburg et al, 2011b) only micromolar amounts of antioxidant polyphenols capable of protecting LDL from oxidation, eventually manage to reach plasma. It was therefore recently proposed that polyphenols from nutrients exert their beneficial effect as antioxidants mainly in the oral cavity and in the stomach (Gorelik et al, 2008; Kanner et al, 2012) but to a much lesser extent in plasma. Accordingly, polyphenols in red wine, coffee, tea, and in other beverages which undergo solubilization by salivary proteins, may now neutralize advanced lipid oxidation end-products generated in the stomach during the metabolism of fatty acids (Gorelik et al, 2008; Kanner et al, 2012) and thus able to prevent the oxidation of LDL in the circulation. It was therefore proposed that consumption of fatty foods be always accompanied by a simultaneous consumption of fruit beverages rich in antioxidant polyphenols. Further research is however needed to assess the role of salivary antioxidants, polyphenols from nutrients, blood elements, and those antioxidants associated with microbial flora as potential players in the homeostasis and in the complex milieu of the oral cavity in health and in disease states.