Abstract
Bovine liver plasma membranes [Rösen, Ehrich, Junger, Bubenzer & Kühn (1979) Biochim. Biophys. Acta587, 593-605] show similar insulin-binding characteristics, as evaluated by Scatchard analysis, to those of membrane systems from other species. However, the dissociation rate of bound insulin cannot be accelerated by the addition of insulin, in contrast with membranes isolated from rat liver. The dissociation rate is strongly dependent on the pH. Although dependent on temperature, the total capacity of binding sites is minimally changed, but the number of high-affinity sites is increased 2-3-fold, by lowering the incubation temperature. These data might be interpreted by assuming a single population of receptors whose distribution between different affinity states depends on temperature. In competition studies, most of the modified insulins examined show a close correlation between binding, determined in plasma membranes from bovine liver, and biological activity, measured in adipocytes. The hypothesis that a positive charge on the A1 residue may be favourable for binding is supported by experiments with an isosteric pair of insulins modified at this residue ([carbamoyl-Gly(A1)]- and [amidino-Gly(A1)]insulin) and with modified insulins carrying one or more positive charges on the A1 residue ([Arg-Gly(A1)]-, [Arg-Arg-Gly(A1)]-, [Arg-Arg-Arg-Gly(A1)]- and [Lys-Arg-Gly(A1)]insulin). The latter insulin derivatives show a higher binding activity for plasma membranes from bovine, porcine and rat liver than expected from their biological activities in adipocytes.