Polyamine Uptake Across the Basolateral Membrane of the Enterocyte Is Mediated by a High-Affinity Carrier

Abstract

Background/Aims: Polyamine uptake from the circulation plays an important role in the maintenance of the intracellular polyamine content during extensive proliferation in intestinal mucosal cells. Methods: Isolated basolateral membrane vesicles of the rabbit enterocyte were used to characterize polyamine transport across the basolateral side of the intestinal epithelium. Incorporation of spermidine and spermine into the basolateral membrane was rapid, although 30–60% of polyamines were initially bound to the basolateral membrane. In order to avoid the influence of binding on the actual uptake into the vesicles, polyamine incorporation was measured at 37 and 4°C, and kinetic parameters were calculated from the difference in polyamine incorporation rates at these temperatures. Results: Uptake kinetics was saturable, with K_m values of 13.34 and 12.35 µmol/l and V_max of 159 and 105 pmol/mg protein/min for spermidine and spermine, respectively. It was also temperature dependent, with Q₁₀ values (calculated between uptake velocities at 37 and 25°C) of 2.56 for spermidine and 1.90 for spermine. At physiological pH, polyamine uptake was at its highest. Since at this pH polyamines are fully charged, charge might be essential for polyamines to be taken up across the basolateral membrane. Polyamine uptake was inhibited by di-, tri- and tetracations, and there was no evidence for sodium cotransport. Transport of putrescine was not inhibited by spermine and spermidine, although spermidine inhibited spermine uptake in a competitive manner, with K_i of 127 µmol/l. Conclusion: These results imply that a saturable high-affinity transport system for polyamine does exist at the basolateral side of the enterocyte. Such a transport system may be responsible for the active transport of polyamine into rapidly proliferating enterocytes.