The absorption of monoacylglycerol by small intestinal brush border me
mbrane is a passive process, i.e., the movement of monoacylglycerol fr
om small unilamellar phospholipid vesicles as donor particles through
the aqueous medium and the incorporation into the outer monolayer of t
he lipid bilayer of the brush border membrane are passive processes in
volving diffusion of the lipid along a concentration gradient. Small u
nilamellar vesicles of egg phosphatidylcholine containing 1 mol % of r
adiolabeled hexadecylglycerol were used as donor, and rabbit small int
estinal brush border membrane vesicles or intact enterocytes isolated
from pig jejunum, as acceptor. Hexadecylglycerol was employed as a lip
ase-resistant model compound for monoacylglycerols. Both acceptor memb
ranes behave similarly in terms of hexadecylglycerol absorption: the k
inetics of hexadecylglycerol absorption are biphasic. The initial fast
phase is due to the movement of hexadecylglycerol from the donor part
icle through the aqueous medium to the outer lipid monolayer of the ac
ceptor 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 a
re relatively large and primarily determined by the high solubility (c
mc) 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 hexadecylglycer
ol into the brush border membrane is rate limiting. The mechanism of t
he hexadecylglycerol absorption involves mainly monomer diffusion and
probably collision-induced transfer.