Lanthanide-stimulated glucose and proline transport across rabbit intestinal brush-border membranes

Cation effects on protein conformation and transport in the Na+/glucose cotransporter

Cation-driven cotransporters are essential membrane proteins in procaryotes and eucaryotes, which use the energy of the transmembrane electrochemical gradient to drive transport of a substrate against its concentration gradient. Do they share a common mechanism? Cation selectivity of the rabbit isoform of the Na+/glucose cotransporter (SGLT1) was examined using the twoelectrode voltage clamp and the Xenopus oocyte expression system. The effect of H+, Li+, and Na+ on kinetics of SGLT1 was compared to the effects of these cations on the bacterial melibiose. In SGLT1, substitution of H+ or Li+ for Na+ caused a kinetic penalty in that the apparent affinity for sugar (K0.5sugar) decreased by an order of magnitude or more (from 0.2 to 30 mM) depending on the membrane potential and cation. The effect of the cation on the K0.5sugar/V profiles was independent of the sugar for glucose and alpha-methyl-beta-D-glucose; this profile was maintained for galactose in Li+ and Na+, but was 2 orders of magnitude higher in H+, but the Imax for glucose, galactose, and alpha-methyl-beta-D-glucose in a given cation were identical. Li+ supported a lower maximal rate of transport (Imax) than Na+ (approximately 80% of ImaxNa), while the Imax in H+ was higher than Na+ (>/=180% of ImaxNa). Our interpretation of these results and simulations using a six-state mathematical model, are as follows. 1) Binding of the cation causes a conformational change in the sugar binding pocket, the exact conformation being determined by the specific cation. 2) Once the sugar is bound, it is transported at a characteristic rate determined by the cation. 3) Mathematical simulations suggest that the largest contribution to the kinetic variability of both cation and sugar transport is associated with cation binding. Similarity to the effects of cation substitution in MelB suggests that the mechanism of energy coupling has been evolutionarily conserved.

Binding of lanthanides to cell membranes in the presence of ligands

The effect of a series of ligands on the binding of the lanthanide, europium (Eu), to rabbit intestinal cell membranes was investigated in vitro. When tested as Eu-ligand complexes (ratio of Eu:ligand, 1:2) of intermediate stability (log stability constant, log K1, for the reaction Eu + L = EuL, of about 7-12) such as Eu-citrate and Eu-nitrilotriacetate (NTA), Eu was available for uptake in a soluble form by intestinal brush-border membrane vesicles (BBMV) in phosphate- and bicarbonate-free solutions at pH 7.2. Ligands with lower log K1 did not maintain Eu in solution whilst those of higher affinity did not donate it to membranes. Generally, there was a clear relationship between log K1 of the Eu-ligand complex and the binding of Eu to BBMV. This relationship identifies ligands that can effectively donate Eu to vesicles under these conditions. BBMV uptake of Eu was due to binding at two sites. Binding to the diethylenetriaminepentaacetate (DTPA)-sensitive site predominated at 20 degrees C and uptake by the DTPA-insensitive site was enhanced at 37 degrees C. Only trace amounts of the bound Eu appeared to be internalized within the vesicles. In the presence of physiological concentrations of phosphate and bicarbonate in cell culture medium, Eu was precipitated from most complexes (at 1:2 and 1:5 Eu:ligand ratio) except DTPA and albumin. Eu precipitation could be prevented by increasing the ligand:Eu ratio. When isolated hepatocytes in cell culture medium were incubated with EuCl3, about 60% of Eu was bound to the cells; Eu-albumin was not bound by hepatocytes.

Expression and characterization of the intestinal Na+/glucose cotransporter in COS-7 cells

Cells derived from the simian kidney, COS-7 cells, were transfected with a eucaryotic expression vector (pEUK-C1) containing the clone for the rabbit intestinal Na+/glucose cotransporter. Expression was monitored after transfection with lipofectin by measuring the initial rate of alpha-methylglucopyranoside (MeGlc) uptake. Cells transfected with vector containing the cDNA for the Na+/glucose cotransporter expressed Na(+)-dependent MeGlc transport. Neither control cells nor cells transfected with vector lacking cloned cDNA expressed the cotransporter. Na(+)-dependent MeGlc uptake into transfected cells was saturable (Km 150 microM), phlorizin-sensitive (Ki 11 microM), and inhibited by sugar analogs (D-glucose greater than MeGlc greater than D-galactose greater than 3-O-methyl-D-glucoside greater than D-allose much greater than L-glucose). Europium was able to mimic Na+ in driving MeGIC uptake. Finally, tunicamycin, an inhibitor of asparagine-linked glycosylation, inhibited the expression of Na(+)-dependent MeGlc transport 80%. We conclude that the rabbit intestinal Na+/glucose cotransporter expressed in COS-7 cell exhibits very similar kinetic properties to that in the native brush border and to that expressed in Xenopus oocytes. In addition, N-linked glycosylation appears to be important for functional expression of this membrane protein.

Endogenous D-glucose transport in oocytes of Xenopus laevis

Endogenous glucose uptake by the oocytes of Xenopus laevis consists of two distinct components: one that is independent of extracellular Na+, and the other one that represents Na+-glucose cotransport. The latter shows similar characteristics as 2 Na+-1 glucose cotransport of epithelial cells: The similarities include the dependencies on external concentrations of Na+, glucose, and phlorizin, and on pH. As in epithelial cells, the glucose uptake in oocytes can also be stimulated by lanthanides. Both the electrogenic cotransport and the inhibition by phlorizin are voltage-dependent; the data are compatible with the assumption that the membrane potential acts as a driving force for the reaction cycle of the transport process. In particular, hyperpolarization seems to stimulate transport by recruitment of substrate binding sites to the outer membrane surface. The results described pertain to oocytes arrested in the prophase of the first meiotic division; maturation of the oocytes leads to a downregulation of both the Na+-independent and the Na+-dependent transport systems. The effect on the Na+-dependent cotransport is the consequence of a change of driving force due to membrane depolarization associated with the maturation process.