The effects of wheat germ agglutinin on the adipocyte insulin receptor.

Treatment of rat adipocytes with wheat germ agglutinin markedly enhances insulin binding by increasing the affinity of the insulin receptor. The structure of adipocyte receptor following wheat germ agglutinin treatment was studied by column chromatography and polyacrylamide gel electrophoresis to determine if aggregates of the insulin receptor are present. Solubilization of the receptor by Triton X-100 and passage of this material over Sepharose 6B revealed two insulin binding activities: a major peak which had a Strokes radius of 87 A and a minor peak with a Strokes radius of 47 A. Wheat germ agglutinin treatment produced an increase in the binding activity of both peaks, but their molecular weights did not change. In addition, 125I-labeled insulin was covalently attached to the adipocyte insulin binding sites by a cross-linking reagent. Column chromatography of the insulin-receptor complex again indicated the presence of two species with Strokes radii of 87 A and 47 A. Heterogeneity in these complexes was also demonstrated by polyacrylamide gel electrophoresis in the presence of Triton X-100. Ferguson plots indicated that the peak of radioactivity had a molecular radius of 60 A, a size found both in the presence and absence of wheat germ treatment of adipocytes. These findings suggest that the increase in receptor affinity produced by wheat germ agglutinin treatment is not caused by the formation of receptor clusters. Instead, it appears that a simple interaction between the plant lectin and the receptor is sufficient to induce the changes in the insulin binding properties of adipocytes.

Effects of wheat germ agglutinin on insulin binding and insulin sensitivity of fat cells.

The plant lectin (wheat germ agglutinin, WGA) produces several alterations in the ability of fat cells to bind and respond to insulin. Although WGA markedly stimulated glucose oxidation, it caused only a modest stimulation of glucose transport. WGA (0.25-20 micrograms/ml) increased the binding of insulin by adipocytes, apparently by increasing the binding affinity of the insulin receptor. With low WGA concentrations (0.25-2.5 micrograms/ml), the elevation in binding was accompanied by an increase in the sensitivity of the adipocytes to insulin stimulation of glucose transport. However, the sensitivity of these cells to vitamin K5 and H2O2 was not altered. With higher WGA concentrations (5-20 micrograms/ml), stimulation of the glucose transport system by insulin, vitamin K5, or H2O2 was markedly inhibited, an effect that is reversed by the addition of ovomucoid. These findings suggest that low WGA concentrations increase the affinity of the insulin receptor and the insulin sensitivity of the cells. At higher concentrations, the lectin appears to act at another site(s) to inhibit the activation of the transport system by insulin or other agents.