The effect of azaserine upon the proline and methyl alpha-D-glucoside transport systems of rat renal brush-border membranes

Characterization of the D-glucose/Na+ cotransport system in the intestinal brush-border membrane by using the specific substrate, methyl alpha-D-glucopyranoside

By using isolated membrane vesicles, we have investigated the tenet that D-glucose transport across the intestinal brush-border membrane involves at least two distinct, Na+-activated agencies (D-glucose transport systems S-1 and S-2), only one of which (S-1) can use methyl alpha-D-glucopyranoside (methyl alpha-glucoside) as a substrate. Our results with this glucose analogue show that: (a) As a function of time, methyl alpha-glucoside uptake exhibits a typical overshoot, similar to but smaller than that given by D-glucose with the same vesicle batch. (b) Nonlinear regression analysis of substrate-saturation curves reveals that, contrary to D-glucose, methyl alpha-glucoside transport involves a single transport system which we have identified as S-1. (c) Methyl alpha-glucoside exhibits an apparent affinity (defined as the reciprocal of Km) 4-times smaller than that of D-glucose for S-1 (Km(Dglucose) = 0.5 mM; Km(methyl alpha-glucoside) = 2 mM). However, methyl alpha-glucoside has a Vmax (230 pmol/mg protein per s) identical to that characterizing D-glucose transport by this system. (d) In the absence of Na+, methyl alpha-glucoside uptake is indistinguishable from simple diffusion, confirming that Na+ is an obligatory activator of S-1. (e) Phlorizin behaves as a fully competitive inhibitor of methyl alpha-glucoside transport (Ki = 18 microM), again indicating that S-1 is involved. (f) Neither phloretin nor cytochalasin B affects methyl alpha-glucoside uptake. We conclude that methyl alpha-glucoside is a substrate specific for S-1, which permits study of the properties of this system without interference by substrate fluxes taking place through any other channel.

The effect of papain upon proline and sodium transport of rat renal brush-border membrane vesicles

Treatment of renal brush-border membrane vesicles with papain resulted in the removal of the activity of maltase, gamma-glutamyl transpeptidase and leucine aminopeptidase by 85, 50 and 75%, respectively. Stripping of these membrane enzyme activities constituted about 2% of the total membrane proteins and resulted in a widespread diminution in the ability of a variety of amino acids and sugars to be taken up by the membrane vesicles which remained osmotically responsive. Kinetic analysis of the uptake of proline, which was shown previously to be transported by both sodium-dependent and sodium-independent systems, revealed that the Vmax for the sodium-dependent system and Km for the sodium-independent system were halved, but other parameters were not affected indicating that the papain treatment altered sodium-gradient-stimulated entry and the affinity of the sodium-gradient-independent system for proline. Experiments on sodium entry and efflux demonstrate a marked enhancement of flux, so that equilibration of the sodium gradient occurred about 5-times more rapidly than in untreated vesicles. This occurred without any change in the osmotic properties of the vesicle with regard to sodium or amino acid uptake. Studies of fluorescence polarization suggest that incubation with papain does not alter the lipid domains of the membrane.