Meeting Abstract
Disaccharide transport across plant membranes has been well-characterized, but only monosaccharides are generally believed to be transported across animal membranes. The present study functionally identified and characterized a disaccharide transporter in lobster (Homarus americanus) digestive tract using purified hepatopancreatic brush border membrane vesicles (BBMV), 14C-radiolabelled sucrose, and a Millipore filtration technique. In the absence of sodium, an acidic pH (e.g. pH 4.0) stimulated sucrose uptake by BBMV significantly (p < 0.05) greater than occurred at pH 5.0, 6.0, or 7.0. At pH 7.0 inside and outside vesicles, a transmembrane Na+ gradient resulted in a 14C-sucrose uptake overshoot at 1 min that was significantly greater than vesicle concentration at equilibrium or in the presence of an inwardly-directed K+ gradient. Experiments using a variety of monosaccharides, disaccharides and trisaccharides, as potential inhibitors of 14C-sucrose uptake, indicated that only maltose and trehalose significantly (one-way ANOVA; p < 0.05) inhibited sucrose transport, suggesting a high degree of transport specificity. Detailed carrier-mediated kinetic analysis of the effects of maltose on 14C-sucrose uptake by BBMV showed that maltose significantly increased sucrose transport Km (control = 0.20 ± 0.06; + maltose = 0.59 ± 0.17 mM) (p < 0.03), without affecting its Jmax (control = 0.58 ± 0.08; + maltose = 0.65 ± 0.13 nmol/mg protein x min) (p > 0.05), suggesting that sucrose and maltose were competitive inhibitors and shared a common transport process. Results suggest that lobster hepatopancreatic BBMV possess an SCRT that is proton, or sodium-dependent, carrier-mediated, and shared by sucrose, maltose, and trehalose, and may function to absorb disaccharides that result from digestion of dietary glycogen and chitin.
