The role of glutathione in the permeation enhancing effect of thiolated polymers

Purpose. To verify or refute the mechanism of permeation enhancement with thiolated polymers via GSH by the use of NaFlu as marker for the paracellular permeation. Methods. The capability of 0.5% polycarbophil cysteine conjugate (PCP-Cys) to reduce 0.02% oxidized glutathione (GSSG) was evaluated via iodometric titration in aqueous solution. Glutathione in its reduced form (GSH; 0.1%-0.4%) and in combination with 0.5% PCP-Cys were tested for their permeation enhancement of sodium fluorescein (NaFlu) and fluorescence labeled bacitracin (bac-FITC) used as paracellular markers. Permeation studies across guinea pig duodenum were carried out in Ussing-type chambers. Opening of the tight junctions was additionally monitored by transepithelial electrical resistance (TEER) measurements. Results. PCP-Cys (0.5%) was shown to reduce 22.0% ± 8.2% of GSSG (0.02%) to GSH in aqueous solution at pH 7.0 and 37°C within 3 h. Permeation of NaFlu was shown to depend on the concentration of GSH. The apparent permeability coefficient (P_app) of NaFlu in buffer only was 4.98 ± 0.5*10^-6, while in the presence of 0.4% GSH a P_app of 9.31 ± 0.92*10^-6 was achieved, representing an enhancement ratio (R = P_app enhancer system/P_app control) of 1.86. The combination of GSH (0.4%) with PCP-Cys (0.5%) led to a significant (p < 0.001) improvement of R for NaFlu up to 2.93 accompanied by a decrease in TEER of 20.3% ± 1.4%. Incubation of bac-FITC with the same GSH / PCP-Cys combination led to an enhancement ratio of 2.06 within 3 h. Conclusion. GSH plays an important role in the opening of tight junctions of intestinal epithelia. It would appear that PCP-Cys is able to reduce GSSG, prolonging the concentration of GSH at the apical membrane, resulting in significantly enhanced paracellular transport.