Dietary triacylglycerol is acted upon by lipolytic enzymes in the stomach and the proximal small intestine, releasing fatty acids and monoacylglycerol as the ultimate products. These digestive products are solubilized by bile released from the gall bladder, resulting in the formation of two product phases-vesicles and micelles-depending upon the concentration of bile in the small intestine. Absorption of lipid is thought to occur from these two phases. We have previously examined the rate and mechanism of long-chain fatty acid transfer between unilamellar vesicles [Kleinfeld, A.M., and Storch, J. (1993) Biochemistry 32, 2053-2061]. In order to begin to assess the relative contributions of micellar vs vesicular phases in the absorption of dietary lipid, a simple model system was designed to investigate the transfer of fatty acid and monoacylglycerol between micelles. A fluorescence self-quenching assay was used to monitor the transfer of fluorescent anthroyloxy-labeled lipids from donor micelles to acceptor micelles. The mechanism of fatty acid transfer was found to be a combination of diffusional and collisional processes, with the latter dominating at high micelle concentrations. The rate of diffusional transfer of fatty acid and monoacylglycerol analogues was approximately 30-fold greater from micelles than vesicles. Intermicellar and intervesicular rates of transfer were 3- fold greater for fatty acids as compared with monoacylglycerol. The results suggest that uptake of the products of intestinal lipase hydrolysis is more efficient from micellar than vesicular phases. Nevertheless, fatty acid and monoacylglycerol transfer from unilamellar vesicles could account, in part, for the relatively efficient uptake of dietary lipid observed in conditions of intestinal bile salt insufficiency.
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