Journal of Lipid Research (Mar 1972)
Stages of uptake and incorporation of micellar palmitic acid by hamster proximal intestinal mucosa
Abstract
The stages of uptake and incorporation of micellar palmitic acid by hamster proximal intestinal mucosa were investigated by incubation of everted sacs at 4°C and 37°C for 2, 5, 10, and 15 min in a micellar solution (10 μmoles of [l-14C]palmitic acid, 10 μmoles of monoolein, and 100μ moles of sodium taurodeoxycholate) and subsequent serial rinsing of the sacs in ice-cold solutions as follows: one 20-sec rinse in unlabeled micellar solution, five 1-min rinses in Krebs-Ringer buffer (0.15 m, pH 6.3), and ten 2-min rinses in 2.5% albumin solution. The fatty acid-solubilizing capacity of all the rinsing solutions was always in excess of the amounts of radioactive palmitic acid released during each rinse. Radioactivity was determined in the tissue homogenates, rinsing solutions, and serosal fluids.The results indicate that a significant proportion of radioactive palmitic acid taken up by the sacs during the short incubation was released into the rinsing solutions. Rinsing in Krebs-Ringer buffer resulted in release of 15.5 ± 2.4% of the labeled fatty acid, and this fraction was independent of the temperature of incubation. In contrast, the amounts of palmitic acid released in albumin were significantly greater and were markedly dependent on the temperature of incubation; a total of 48.6 ± 7.0% and 26.3 ± 5.1% was released from sacs incubated at 4°C and 37°C,respectively. While the proportion of radioactive palmitic acid in the free fatty acid fraction of the tissue after the rinsing sequence remained reasonably constant regardless of the temperature and duration of incubation, the radioactivity of the esterified palmitic acid in the tissue was much greater in the sacs incubated at 37°C and tended to increase linearly up to 10 min of incubation. A highly significant inverse relationship was found between the fraction of radioactive palmitic acid released by rinsing in albumin and the fraction of the label in the tissue esterified fatty acids.The results suggest that the initial uptake of micellar fatty acid by intestinal mucosa may involve reversible binding to superficial sites with at least two strengths of binding: a weak, temperature-independent binding which could be easily dissociated by rinsing in Krebs-Ringer buffer, and a stronger, temperature-dependent binding which could be dissociated by rinsing in albumin, but not in Krebs-Ringer buffer. Analogous binding of micellar palmitic acid occurred in a brush border preparation of proximal intestine which was devoid of any fatty acid esterifying activity. This suggested that the reversible binding of fatty acid by the intestinal mucosa may be a property of its superficial components, namely the glycocalyx or microvillous membranes, and that it may be independent of the esterifying capacity of the tissue.
