Abstract
Binding of insulin to islets of Langerhans was studied. It was found that "specific" binding of [125I]insulin ("specific" binding equals total binding minus nonspecific binding) was saturable with respect to time and insulin concentration and depended on the number of incubated islets. Furthermore, bound insulin was displaced by native insulin in a dose-dependent manner. Bound [125I]insulin was easily dissociated and there was little [125I]insulin degradation both in the incubation medium and during the processes of binding and dissociation. Scatchard analysis of experiments with increasing [125I]insulin concentration and with displacement of insulin binding by native insulin revealed "high affinity" binding sites with a dissociation constant of 0.461 +/- 0.08 n M and 3.5 X 10(6) high affinity binding sites per islet. There also existed "low affinity" binding sites with dissociation constant (Kd) of 43.9 +/- 11.6 nM and 5.9 X 10(7) low affinity binding sites. High affinity binding sites of islets from rats pretreated with alloxan decreased by about one half, whereas Kd was unaffected. Because the Kd of specific high affinity binding and mean effective dose (ED50) of the biological effects of insulin on normal pancreatic islets are in the same range (between 0.46 and 1.19 nM), the insulin-receptor interaction may be biologically significant.