Absorption of monoacylglycerols by small intestinal brush border membrane

The absorption of monoacylglycerol by small intestinal brush border membrane is a passive process, i.e., the movement of monoacylglycerol from small unilamellar phospholipid vesicles as donor particles through the aqueous medium and the incorporation into the outer monolayer of the lipid bilayer of the brush border membrane are passive processes involving diffusion of the lipid along a concentration gradient. Small unilamellar vesicles of egg phosphatidylcholine containing 1 mol% of radiolabeled hexadecylglycerol were used as donor, and rabbit small intestinal brush border membrane vesicles or intact enterocytes isolated from pig jejunum, as acceptor. Hexadecylglycerol was employed as a lipase-resistant model compound for monoacylglycerols. Both acceptor membranes behave similarly in terms of hexadecylglycerol absorption: the kinetics of hexadecylglycerol absorption are biphasic. The initial fast phase is due to the movement of hexadecylglycerol from the donor particle through the aqueous medium to the outer lipid monolayer of the acceptor membrane, and the second slow phase probably involves the flip-flop motion of hexadecylglycerol from the outer to the inner monolayer of the acceptor membrane. The values for the pseudo-first-order rate constants of the initial fast phase for hexadecylglycerol absorption are relatively large and primarily determined by the high solubility (cmc) of hexadecylglycerol in aqueous media. The pseudo-first-order rate constants depend linearly on the protein (lipid) concentration of the acceptor membrane, indicating that the on rate of the hexadecylglycerol into the brush border membrane is rate limiting. The mechanism of the hexadecylglycerol absorption involves mainly monomer diffusion and probably collision-induced transfer.