Understanding interactions of mucins with flavour compounds in tea

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Abstract

Mucin glycoproteins are the structural components of mucus barriers throughout the body. The polymeric gel forming mucins form disulphide bonded networks that govern mucus viscoelasticity and thus its protective barrier properties. Recently, there has been considerable interest in the interactions between the mucus barrier and dietary components in terms of the bioavailability of beneficial molecules. Black tea theaflavins are considered potent antioxidants and have anticancer properties however, their bioavailability is low. The consumption of tea influences mouthfeel, causing dry, puckering sensation known as astringency. The interactions between tea polyphenols and components of the mucus barrier, including mucins, have been shown to be responsible this mouthfeel. Previous studies have focused on the green tea catechins, epicatechin (EC) and epigallocatechin-3-gallate (EGCG) and have shown that EGCG facilitates mucin cross-linking resulting in their aggregation. Additionally, these studies have shown that EC does not cause mucin reorganisation to the same extent, concluding that the galloyl moiety is essential in this interaction. Here we investigated the properties of the salivary mucins in the presence of black tea theaflavins, aiming to elucidate whether they are able to facilitate the same interactions as the green tea catechins. We utilised electrophoretic and centrifugal separations to analyse the effect of theaflavin interaction with MUC5B and its subdomains. Results suggest that there is a dramatic reorganisation of the MUC5B network in, and purified from, human whole saliva upon addition of the galloylated theaflavins. Infra-red analysis of the theaflavins in the presence of MU5B suggested that the galloyl group of the theaflavin is essential in mediating mucin-polyphenol interactions. Utilising recombinant mucin protein domains; N-terminal, C-terminal and Cys7 we aimed to elucidate the site of theaflavin interaction. We show that the theaflavins interact with different regions of the mucin with differing affinities and that this interaction is influenced by conditions such as pH and cation content. Furthermore, milk is commonly added to black tea and is shown to decrease the perception of astringency. Using the same methods we show that the milk protein β-lactoglobulin is able to sequester tea polyphenols which prevents mucin aggregation. From this study we are able to build upon a previously suggested model of polyphenol-mucin interactions. We conclude that the N-terminal and Cys-rich regions of MUC5B associates first with the theaflavin further cross-linking the mucin polymers. As the local concentration of the galloylated theaflavin increases they interact with C-terminal thus leading to the production of insoluble mucin aggregates.

Keyword(s)

MUC5B; Mucin; Saliva; Tea; Theaflavin

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