Experimental and Theoretical Studies on the Binding of Epigallocatechin Gallate to Purified Porcine Gastric Mucin

Abstract

Binding of epigallocatechin gallate (EGCG) to highly purified short side-chain porcine gastric mucin similar to human MUC6 type has been studied by ultraviolet-visible absorption spectroscopy (UV-vis), ultrafiltration isothermal titration microcalorimetry (ITC) and transmission electron microscopy (TEM). The thermodynamic equilibrium of EGCG binding to mucin has been quantitatively determined using ultrafiltration and high-performance liquid chromatography (HPLC)-UV/vis. The relationship suggests multilayer binding rather than simple Langmuir monolayer binding of EGCG. By combining the ultrafiltration and ITC data, the thermodynamic parameters of EGCG binding to mucin have been obtained. The binding constant for the first layer is about an order of magnitude higher than that of the consecutive multilayers. Negative entropy indicates multilayer of EGCG formed. Hydrogen bonding may be responsible for the multilayer formation. Increasing temperature resulted in a decrease in the binding affinity, further suggesting that hydrogen bonds dominated the interaction energy. A TEM micrograph of the EGCG-mucin complex revealed a monodispersion of blobs similar to pure mucin solution but with relatively bigger size (about twice). It is proposed that the EGCG-mucin binding process occurs by single and/or cluster of EGCG molecules driven to the surface of the two hydrophobic globules of mucin by hydrophobic interaction followed by hydrogen bond interaction between EGCG and mucin. Further adsorption of EGCG molecules onto bound EGCG molecules to form multilayers can also occur. This fits well with the observations that EGCG-mucin interaction followed a multilayer adsorption isotherm, the energy released is dominated by hydrogen bonds, and no large aggregates were formed.