Functional characterization of a putative disaccharide membrane transporter in crustacean intestine
Transepithelial absorption of dietary sucrose in the American lobster, Homarus americanus, was investigated by mounting an intestine in a perfusion chamber to characterize mucosal to serosal (MS) 14C-sucrose transport. These fluxes were measured by adding varying concentrations of 14C-sucrose to the perfusate and monitoring their appearance in the bathing solution. Transepithelial 14C-sucrose transport was the combination of a hyperbolic function of luminal concentration, following Michaelis–Menten kinetics, and apparent diffusion. The kinetic constants of the putative sucrose transporter were KM = 20.50 ± 6.00 µM and Jmax = 1.81 ± 0.50 pmol/cm2 × min. Phloridzin, an inhibitor of Na+-dependent mucosal glucose transport, decreased MS 14C-sucrose transport. Decreased MS 14C-sucrose transport also occurred in the presence of luminal trehalose, a disaccharide containing d-glucose moieties. Thin-layer chromatography (TLC) identified the chemical nature of radioactively labeled sugars in the bath following transepithelial transport. TLC revealed 14C-sucrose was transported across the intestine largely intact with no 14C-glucose or 14C-fructose appearing in the serosal bath or luminal perfusate. Only 13 % of bath radioactivity was volatile metabolites. Results suggest that disaccharide sugars can be transported intact across crustacean intestine and support the occurrence of a functional disaccharide membrane transporter.