Isolation and properties of the mixed lipid micelles present in intestinal content during fat digestion in man.

To evaluate better the physicochemical characteristics of human fat digestion, a method was developed which allowed characterization of the bile acid-lipid mixed micelles of the aqueous phase of post-prandial duodenal fluid. Duodenal fluid was collected after a 36-g fat breakfast for two 90-min periods and for 60 min after i.v. cholecystokinin and was ultracentrifuged at 15,400,000 g-min. The aqueous phase was isolated, passed through a 200-nm filter, and the mixed micelles were concentrated by an ultrafiltration procedure using a 1.5-nm filter. The 1.5-nm retentate was eluted from Sepharose 6B columns with 1.5-nm filtrate for both preequilibration fluid and eluent. 1.5-nm filtrate approximated the monomer concentrations. Each sample was assayed for bile acid, fatty acid, lecithin, lysolecithin, protein, cholesterol, and counterions (pH, Na+, K+, Ca2+). Constituents were concentrated only on the 1.5-nm filter. On gel permeation chromatography, coincident peaks were observed for bile acid, fatty acid, lysolecithin, and cholesterol; and were eluted with a Kav range of 0.50-0.68 (corresponding to a Stokes radius of 2.3-3.5 nm). An average density of 1.25 and coincident peaks of bile acid and fatty acid were found for the mixed micelles on sucrose density gradients. The regression lines of micellar fatty acid, lysolecithin, and cholesterol vs. bile acid gave a stoichiometry of 1.4 mol fatty acid, 0.15 mol lysolecithin, and 0.06 mol cholesterol for each mole of bile acid. Mixed micelles were homogeneous in composition. These results provide direct evidence for the existence of the postprandial mixed micelle and describe several of its physicochemical properties.