AJP - Gastrointestinal and Liver Physiology, Vol 239, Issue 3 177-G182, Copyright © 1980 by American Physiological Society
pH dependence of micellar diffusion and dissociation
Y. F. Shiau and G. M. Levine
A simplified model system was used to examine the effect of pH on the
diffusion and dissociation of mixed micelles across an interphase.
Diffusion of radioactively labeled taurocholate and oleic acid from a
micellar solution into a phosphate buffer of varying pH (5.0-8.0) was
measured. The proportion of bile salt and fatty acid as monomers and
aggregates in both solutions was determined by ultrafiltration. Results
indicate that all of the oleic acid existed as aggregates in the micellar
solution, whereas taurocholate was in equilibrium between aggregate and
monomer forms. Between pH 5.0 and 7.0, oleic acid and taurocholate
diffusion was inversely related to pH and due to the differences in
aggregate but not monomer diffusion. Fatty acid diffusion across the
interface was always less than taurocholate diffusion, because taurocholate
monomers could also diffuse across the interphase. Calculation of the
proportion of taurocholate diffusion from aggregate and monomers based on
micellar composition was similar to measured values at pH greater than or
equal to 6.5, indicating a lack of micelle dissociation. However, at pH
less than or equal to 6 measured values for bile salt monomers exceeded
calculations, indicating that a low pH microclimate favors dissociation of
micelles. A hypothesis is advanced to explain these events occurring in a
disequilibrium area adjacent to the epithelial cell membrane.
