Acidic mucin layer facilitates micelle dissociation and fatty acid diffusion.

The presence of a mucin layer on the surface of the intestinal epithelium has been suggested as an important factor in maintaining an acidic microclimate. The presence of such a low-pH compartment has been shown to facilitate fatty acid uptake. The mechanisms leading to the enhancement of fatty acid uptake were investigated in a purified acidic mucin layer. Our results indicate that the presence of a low-pH compartment indeed facilitates the dissociation of mixed micelles made of taurocholate and oleic acid. The released fatty acid formed an emulsion at the mucin layer, and this event could be visualized by the naked eye. When the size of the particles in the micelle solution was examined by photon correlation spectroscopy, it was found that acidification alone can lead to the formation of particles with size substantially greater than that of micelles. With the use of labeled fatty acid, the change in optical density can be correlated to the amount of fatty acid appearing in the mucin layer in an asymptotic fashion, suggesting that using the turbidity as an indicator might underestimate fatty acid diffusion. Despite this limitation, the rate of fatty acid diffusion in the mucin layer was estimated to be 400% of that in the buffer solution.