Cupric ion-mediated active transport of amino acids in membrane vesicles of Mycobacterium phlei

Classical and channel-like urate transporters in rabbit renal brush border membranes

The precise mechanism by which urate is transported across rabbit renal proximal tubule luminal membranes has not been defined. To determine whether urate flux across this membrane represents simple diffusion or transport on specific carriers, urate uptake was examined in brush border membrane vesicles that were prepared by a Mg+(+)-aggregation technique and then exposed to CuCl2. Na(+)-independent, voltage sensitive urate transport was demonstrated in these Cu+(+)-exposed vesicles. Transport was trans-stimulated by urate and cis inhibited by pyrazinoic acid and oxonate. A small fraction of transported urate and urate in the extravesicular fluid was oxidized to allantoin. Kinetic analysis revealed the presence of two kinetically distinct transporters; a channel-like carrier that was inhibited by pyrazinoic acid and oxonate, and a high-affinity, classical, saturable carrier that was inhibited by higher concentrations of oxonate. These studies provide the first direct evidence for carrier-mediated urate transport in rabbit renal brush-border membranes and demonstrate that the rabbit transporter(s) share a number of properties with the urate uniporter in rat proximal tubule cell membranes.

Sulfhydryl group involvement in the modulation of neutral amino acid transport in thymocyte membrane vesicles

Membrane vesicles from rat thymocytes accumulate 2-aminoisobutyric acid in the presence of 0. 1 M NaCl. Uptake is half maximal between 15 and 30 seconds after addition of the amino acid and reaches a plateau value after about 2 minutes. The uptake of 2-aminoisobutyric acid can be modulated by various sulfhydryl reagents. Reduced glutathione leads to an inhibition of uptake whereas oxidized glutathione increases uptake. Agents such as insulin and diamide which can induce disulfide formation lead to an activation of transport. These date indicate that uptake of the Na+-dependent amino acid, 2-aminoisobutyric acid, in thymocytes is modulated by a putative plasma membrane, sulfhydryl-containing protein.