Intercation of soduim taurodeoxycholate with dipalmitoyllecithin—cholesterol mixed monolayers

Abstract

The penetration process of sodium taurodeoxycholate in dipalmitoyllecithin—cholesterol mixed monolayers has been studied at the air—water interface, following two different methodologies: one at constant area and the other at constant pressure. In the constant area method the variation of the surface pressure of the mixed monolayers has been determined by measuring the different molar fractions of dipalmitoyllecithin and cholesterol after injection in the aqueous subphase of different amounts of sodium taurodeoxycholate. The rate of the penetration process increases with the amount of biliar salt injected and with the proportion of cholesterol in the mixed monolayer. The initial surface pressure of the lipid monolayer also notably influences the process rate. It increases with an increase in the initial pressure to a certain level, depending on the composition of the mixed monolayer, after which, with an increase in the initial pressure, the speed of the process decreases. These results are explained in terms of the bile salt penetration being an active process favoured by the presence of lipid molecules in the monolayer. The study at constant surface pressure has not shown any linear area—time relationship on observing the surface occupied by the monolayer after the injection of sodium taurodeoxycholate beneath lecithin—cholesterol mixed monolayers. The penetration rate is increased as in the case of constant area, on increasing the amount of the injected biliar salt and the proportion of cholesterol in the mixed monolayer. Likewise, there exists a surface pressure at which the speed is a maximum. A quantitative kinetic interpretation of the experimental results is given for both constant area and constant pressure.