Different effects of concanavalin A on insulin binding to cultured cells in monolayers: "blocking" and "trapping" effects of concanavalin A.

Abstract

The effect of concanavalin A on insulin binding was studied in cells cultured in a monolayer. Preliminary incubation of concanavalin A with Reuber rat hepatoma cells ( R117 - 21B ) showed an inhibitory effect on insulin binding at 25 degrees C and 4 degrees C; a "blocking" effect. But, even at high concentrations (greater than 50 micrograms/ml), concanavalin A could not completely inhibit the specific binding of insulin to these cells. Under these conditions, the number of high affinity insulin-binding sites decreased when compared to the number of sites in the control. In contrast, when cells first were incubated with insulin then with insulin plus concanavalin A, the binding of insulin increased, even at high concentrations of the lectin. This is a "trapping" effect. This effect increased the number of insulin-binding sites as compared to the number in the control. The apparent equilibrium constant of insulin for the cells was changed slightly by the concanavalin A in R117 - 21B cells. The binding of concanavalin A to these cells also was investigated. Insulin did not affect its binding to the cells. This study shows that insulin-binding sites can be classified into two groups in which insulin binding is affected by concanavalin A or is independent of the lectin. The "blocking" effect of concanavalin A on insulin binding suggests that the binding sites of insulin and concanavalin A on the insulin receptors are very close.