Influence of sodium ions on detergent solubilization of pig brush border D-glucose transport system for reconstitution experiments

Use of mild detergent gel electrophoresis for isolation and characterization of the kidney brush border D-glucose transporter

Gradient gel electrophoresis was performed under mild detergent conditions to separate pig kidney brush border membrane proteins and to identify the smallest functional molecular protein entity of the D-glucose transporter. The various protein bands obtained from the nondenaturing gel system in a semipreparative scale were eluted by electrodialysis. These proteins were then reintegrated into proteoliposomes and tested for D-glucose-inhibitable [3H]phlorizin binding. The D-glucose transporter had a molecular mass of 70 kDa in mild detergent electrophoresis conditions and in subsequent SDS analysis.

Separation and reconstitution of sodium-dependent glucose transport activity from renal brush-border membranes using gel-filtration chromatography

Pig kidney brush-border membrane vesicles were solubilized using a final concentration of 1% Triton X-100, found optimal for quantitative reconstitution of D-glucose transport into liposomes. Using reconstituted proteoliposomes, selective permeability towards D-glucose compared to other sugars tested was shown as well as the main features of D-glucose transport in native membranes, namely sodium dependence and phlorizin inhibition of D-glucose accumulation. After removal of Triton X-100 from the detergent extract, some membrane proteins (about 40%), which are insoluble in the absence of detergent, were isolated. Among these proteins resolubilized by 1% Triton X-100, the component catalyzing the D-glucose transport was located by gel-filtration chromatography separation, using reconstitution of transport as the assay. The active fraction displayed a molecular size of 50 A; when analyzed on SDS polyacrylamide gel electrophoresis, it contained one major protein subunit with an apparent molecular weight close to 65,000. We conclude that this protein fraction is involved in D-glucose transport by renal brush borders.

Reconstitution of brush border membrane proteins in phosphatidylcholine vesicles. Biochemical and functional characterization

Horse kidney brush border membrane proteins were incorporated into phosphatidylcholine vesicles. Structural analysis of proteoliposomes prepared with various lipid:protein ratios showed that: (a) only a few of the proteins present in the crude brush border extract are integrated, (b) all known membrane hydrolases are integrated, and (c) these proteoliposomes are homogeneous vesicles. Papain solubilization of brush border membrane hydrolases, i.e. aminopeptidase M, neutral alpha-glucosidase, gamma-glutamyltransferase and alkaline phosphatase, performed in parallel on native membrane vesicles and proteoliposomes, revealed similar kinetics. Analysis of membrane vesicles and proteoliposomes on sucrose density gradients either without any treatment, or after papain treatment showed that: (a) in proteoliposomes, neutral alpha-glucosidase is associated with radiolabelled phosphatidylcholine, and (b) papain-treated vesicles and proteoliposomes released enzyme activity in the same way. These results suggest that the integration mechanism of brush border membrane proteins may be similar in proteoliposomes and native membrane vesicles. Transport experiments under equilibrium exchange conditions showed that the uptake properties of proteoliposomes are similar to those of brush border membrane vesicles.