Spectral properties and ionization behavior of retinoids, II. Bile salt solutions

Purpose: The spectral properties and ionization behavior of retinoic acid (RA) and three structurally related arotinoids, MTTO, TTNPB, and TTNN, have been determined in simple micellar and mixed micellar solutions to provide a better understanding of the intestinal absorption of retinoids. Methods: Spectrophotometric and pH measurements have been made to determine the ionization constant in bile salt solutions. The fluorescent intensity and polarization of TTNN was determined. The extent of solubilization of retinoic acid in 10 mM NaTC/10 mM egg PC solutions was determined as a function of pH. Results: The rank order of wavelengths of maximum absorption, lambda(max), was as follows: aqueous solution > dihydroxy bile salt > trihydroxy bile salt > ethanol > mixed micelles. The interaction as reflected in the lambda(max) of arotinoids with the bile salts at low pH depended on the mixing order, but this was not the case for retinoic acid. The rank order of the observed negative logarithm of the ionization constants, pK(aobs), was aqueous solution > mixed micelles > dihydroxy bile salt > trihydroxy bile salt which reflects both the polarity as well as the electrostatic charge of the aggregates. The calculated electrostatic and dielectric effects on the ionization constant were comparable in bile salt micelles suggesting that TTNN is repositioned with a change in the ionic strength and that the size distribution of bile salt simple micelles appears to be perturbed by the presence of TTNN. In addition, the size of the bile salt-TTNN aggregate was independent of ionic strength and the type of bile salt. The solubilization of retinoic acid in bile salt/egg PC mixed micelle does not follow the expected dependence of pH indicating the presence of specific interactions. Conclusions: Retinoic acid and its derivatives exist almost exclusively in aggregated forms with RA forming a distinct type of aggregate in comparison to the arotinoids. The values of the pK(aobs) of the retinoids ranged from near 5 in simple bile salt aggregates to >7 in self-associated aggregates and mixed micelles which reflects the sensitivity of the ionization to the environment. This sensitivity has direct implications for the extent solubilized in the intestine and thereby complicates efforts at understanding the mechanism of the oral absorption of retinoids. (C) 1998 Published by Elsevier Science B.V. All rights reserved.