FATTY-ACID TRANSFER IN TAURODEOXYCHOLATE MIXED MICELLES

Dietary triacylglycerol is acted upon by lipolytic enzymes in the stom ach and the proximal small intestine, releasing fatty acids and monoac ylglycerol as the ultimate products. These digestive products are solu bilized by bile released from the gall bladder, resulting in the forma tion of two product phases-vesicles and micelles-depending upon the co ncentration of bile in the small intestine. Absorption of lipid is tho ught to occur from these two phases. We have previously examined the r ate and mechanism of long-chain fatty acid transfer between unilamella r vesicles [Kleinfeld, A. M., & Storch, J. (1993) Biochemistry 32, 205 3-2061]. In order to begin to assess the relative contributions of mic ellar vs vesicular phases in the absorption of dietary lipid, a simple model system was designed to investigate the transfer of fatty acid a nd monoacylglycerol between micelles. A fluorescence self-quenching as say was used to monitor the transfer of fluorescent anthroyloxy-labele d lipids from donor micelles to acceptor micelles. The mechanism of fa tty acid transfer was found to be a combination of diffusional and col lisional processes, with the latter dominating at high micelle concent rations. The rate of diffusional transfer of fatty acid and monoacylgl ycerol analogues was approximately 30-fold greater from micelles than vesicles. Intermicellar and intervesicular rates of transfer were 3-fo ld greater for fatty acids as compared with monoacylglycerol. The resu lts suggest that uptake of the products of intestinal lipase hydrolysi s is more efficient from micellar than vesicular phases. Nevertheless, fatty acid and monoacylglycerol transfer from unilamellar vesicles co uld account, in part, for the relatively efficient uptake of dietary l ipid observed in conditions of intestinal bile salt insufficiency.