Inactivation of the renal outer cortical brush-border membrane D-glucose transporter by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline

Effect of arginine modification on K(+)-dependent leucine uptake in brush-border membrane vesicles from the midgut of Philosamia cynthia larvae

The effect of phenylglyoxylation on the midgut K(+)-dependent leucine transport was studied using lepidopteran brush-border membrane vesicles. The inhibition of leucine uptake by phenylglyoxal (PGO) showed a biphasic inactivation pattern. The second-order rate constant for the slow and fast phases were 0.0020 mM-1 min-1 and 0.0091 mM-1 min-1, respectively. However, substitution of borate buffer for Hepes-Tris buffer produced a mono-exponential inactivation pattern, suggesting modification of a single arginine group. The effect of PGO was dose-dependent and the concentration causing half-maximal inhibition of leucine uptake was 5.1 +/- 0.3 mM. Leucine transport was significantly inhibited also in the absence of a potassium electrochemical gradient (i.e., [K+]in = [K+]out = 100 mM), suggesting that inhibition was not related to a decrease in the driving force. Moreover, intravesicular volume remained unchanged after preincubation with PGO. Kinetic analysis of the interaction of PGO with the leucine cotransporter revealed that (i) inhibition was related to a decrease in the Vmax value and (ii) neither leucine nor K+ were able to prevent the inhibition. Our results suggest an important role for arginine residues in the molecular mechanism of K+/leucine cotransport in lepidopteran larvae midgut.

Some characteristics of sodium-independent phosphate transport across renal basolateral membranes

Sodium-independent phosphate transport was evaluated in porcine renal basolateral membrane vesicles. Phosphate uptake was saturable with an apparent Km 10.1 +/- 1.2 mM and Vmax 13.6 +/- 2.0 nmol (mg protein)-1 min-1, n = 5. Phosphate uptake was trans-stimulated with intravesicle phosphate and was enhanced with a positive transmembrane electrical potential. Arsenate and bicarbonate inhibited phosphate transport but other anions including sulfate and phosphonoformate were without effect. These studies indicate that phosphate uptake across basolateral membranes is present in the absence of sodium, is facilitated, and is specific for phosphate. The apparent affinity and rate of phosphate transport across the basolateral membrane is significantly higher than the respective parameters observed for the brush-border membrane.

Inhibition of Na+-dependent phosphate transport by group-specific covalent reagents in rat kidney brush border membrane vesicles. Evidence for the involvement of tyrosine and sulfhydryl groups on the interior of the membrane

The effects of tyrosine- and sulfhydryl-specific reagents on the Na+-dependent transport of phosphate in brush border membrane vesicles prepared from rat renal cortex were investigated. This study is the first to show that the tyrosine-specific reagents 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole and tetranitromethane inactivate the transporter in a concentration- and time-dependent fashion while the membrane impermeant tyrosine reagent, N-acetylimidazole, has no effect on phosphate uptake. The membrane permeant sulfhydryl reagent N-ethylmaleimide also caused a time- and concentration-dependent inactivation of this transport process but the membrane impermeant reagents 7-chloro-4-sulfobenzo-2-oxa-1,3-diazole and eosin-5-maleimide had little effect on phosphate uptake. The inhibitory effects of both tyrosine- and sulfhydryl-specific reagents were additive, but no protection from inactivation by tyrosine-specific reagents could be achieved by preincubation of the vesicles with the substrates of the transporter or with competitive inhibitors of the transport process. These results suggest that the amino acids modified by these agents are located either within the membrane or on the cytosolic surface of the transporter. These residues may not participate in substrate binding, but may be important for the conformational change of the transporter necessary for the translocation of phosphate across these membranes. This study also shows that Na+-dependent phosphate transport can be inactivated by other reagents which covalently modify histidine, carboxyl, and amino groups on proteins.

Leucine transport system in a facultatively alkalophilic Bacillus

Some characterizations of the leucine transport system in a facultative alkalophile, which is able to grow over a wide pH range from 7.0 to 10.5, were attempted. Although the direction of a transmembrane pH gradient of the bacterium below pH 8.2 is opposite to that above pH 8.2 (N. Koyama and Y. Nosoh (1985) Biochim. Biophys. Acta 812, 206-212), leucine transport is likely to be driven only by sodium electrochemical potential irrespective of the external pH. It was suggested that histidine and sulfhydryl groups in the leucine transporter are involved in the translocation mechanism and the pK value of the histidine residue involved is approximately 7.0.

Inhibition of sodium-dependent transport systems in rat renal brush-border membranes with N,N'-dicyclohexylcarbodiimide

Treatment of rat renal brush-border membrane vesicles with N,N'-dicyclohexylcarbodiimide (DCCD) causes irreversible inhibition of the Na+-coupled transport systems for D-glucose, L-phenylalanine, L-glutamate, and sulfate. The DCCD-reactive side groups of these transport systems differ in their sensitivity towards DCCD and protection by substrates. The D-glucose and L-glutamate transporters cannot be protected by their substrates. In contrast, Na+ protects the transport systems for L-phenylalanine and sulfate from inactivation by DCCD. The data suggest covalent modification by DCCD of D-glucose and L-glutamate transporters apart from their substrate binding sites and of L-phenylalanine and sulfate transporters within their Na+-binding regions.

Diethylpyrocarbonate inhibition of sodium-glucose cotransport in kidney brush-border membrane vesicles

Exposure of kidney brush-border membrane vesicles to the acylating reagent diethylpyrocarbonate resulted in inactivation of the glucose transporter, as demonstrated by inhibition of sodium-coupled D-glucose transport and phlorizin binding. The transport site(s) was protected against inactivation by the simultaneous presence of sodium ions and D-glucose, and were partially protected by phlorizin. Transport activity was not restored by hydroxylamine; this rules out the possibility of diethylpyrocarbonate interaction with histidine, serine or tyrosine transporter residues. Dithiothreitol, a thiol protector, slightly prevented diethylpyrocarbonate inactivation. It is therefore suggested that (an) amino group(s) in the translocation complex is involved, at the level of the sugar transport site and the preferential protection of D-glucose against diethylpyrocarbonate inactivation related to a conformation change caused by the simultaneous binding of sodium and D-glucose to the cotransporter.

Critical carboxyl group(s) in Na+-dependent cotransporters of the intestinal brush-border membrane

We have previously provided functional evidence for a role of carboxyl group(s) in the mechanism of coupling of Na+ and D-glucose fluxes by the small-intestinal cotransporter(s) (Kessler, M. and Semenza, G. (1983) J. Membrane Biol. 76, 27-56). We present here a study on the inactivation of the Na+-dependent transport systems, but not of the Na+-independent ones, in the small-intestinal brush-border membrane, by hydrophobic carbodiimides. Although marginal or insignificant protection by the substrates or by Na+ was observed, the parallelism between Na+-dependence and inactivation by these carbodiimides strongly indicates the role of carboxyl group(s) previously indicated. Contrary to the carboxyl group identified by Turner [1986) J. Biol. Chem. 261, 1041-1047) in the sugar binding site of the renal Na+/D-glucose cotransporter, the carboxyl group(s) studied here probably occur elsewhere in the cotransporter molecule.