pH dependence of micellar diffusion and dissociation

Area of Science:

• Biochemistry
• Physical Chemistry
• Cell Biology

Background:

• Mixed micelles are crucial for lipid absorption in the gastrointestinal tract.
• The behavior of micelles, including their diffusion and dissociation, can be influenced by environmental factors like pH.
• Understanding micelle dynamics at the epithelial cell membrane is vital for comprehending nutrient transport.

Purpose of the Study:

• To investigate the impact of varying pH on the diffusion and dissociation of mixed micelles (bile salt and fatty acid).
• To determine the relationship between pH, micelle composition (monomers vs. aggregates), and diffusion rates across an interphase.
• To explore micelle dissociation mechanisms in a simplified model relevant to epithelial cell membranes.

Main Methods:

• Utilized a simplified model system with radioactively labeled taurocholate and oleic acid.
• Measured diffusion rates of micellar components into phosphate buffers ranging from pH 5.0 to 8.0.
• Employed ultrafiltration to quantify the proportions of monomers and aggregates in solution.

Main Results:

• Oleic acid existed solely as aggregates, while taurocholate was in equilibrium between aggregate and monomer forms.
• Diffusion rates of both oleic acid and taurocholate were inversely related to pH between 5.0 and 7.0.
• Lower pH (≤ 6) led to increased diffusion of bile salt monomers, indicating micelle dissociation, contrary to higher pH (≥ 6.5).

Conclusions:

• A low pH microenvironment adjacent to the epithelial cell membrane appears to favor micelle dissociation.
• Differences in aggregate and monomer diffusion, influenced by pH, significantly affect transport across the interphase.
• The findings support a hypothesis explaining events in the disequilibrium area near the epithelial cell membrane during lipid absorption.