Identifying the Key Protein-Protein Interactions in Salivary Delubrication

Objectives: Characterising salivary protein function in maintaining oral lubrication is key to understanding xerostomia and food-induced astringency (perception of dry mouth). Xerostomia is not just insufficient saliva production, as sensation of mouth dryness can occur despite the presence of saliva. Our group has recently demonstrated that sodium ions are crucial to salivary lubrication due to the hydration shells surrounding the sodium ions. In the present study we also show sodium ions are crucial to protein-protein interactions that can lead to salivary film delubrication. This mechanism is distinct and separate to aggregation-induced delubrication as occurs when polyphenols cause astringency.
In the present study we identify a mechanism in salivary delubrication caused by food proteins and determine the significance of sodium in protein-protein interactions.
Methods: Protein-protein interactions were defined using native SDS-PAGE, size exclusion chromatography and mass photometry. Salivary lubrication was determined using a PDMS-PDMS tribopair using a Bruker tribometer. Samples were de-ionised by centrifugal dialysis.
Results: Using native SDS-PAGE salivary protein-food protein interactions increased in the presence of sodium chloride (NaCl) (P<0.05) compared to without. Tribological results also demonstrated that astringents (pea protein, casein etc.,) increased the friction coefficient of saliva (by X4) and that this interaction was decreased by the absence of sodium.
Conclusions: Sodium ions bound to glycosylated salivary proteins are important to salivary lubrication but can also be a target for food proteins. Thus, sodium appears to enhance protein-protein interactions although this alone cannot explain salivary delubrication. Further research should explore the clinical application of sodium-induced protein-protein interactions affecting hydration theory lubrication defining the underlying mechanism of xerostomia and methods to improve oral dryness.