Specific binding of nonsupressible insulinlike activity to chicken embryo fibroblasts and to a solubilized fibroblast receptor.

¹²⁵I-labeled nonsuppressible insulinlike activity—soluble in acid/ethanol (NSILA-S) binds to cultured chicken embryo fibroblasts and to an extract obtained by treating the fibroblasts with Triton X-100. Binding to intact cells and to the cell extract is timetemperature, and pH-dependent and shows saturation kinetics. The apparent intrinsic association constant for binding of NSILA-S to intact cells is 10⁹ M^?1, the number of binding sites per cell approximately 6000. Cold NSILA-S preparations of different purity displace bound ¹²⁵I-labeled NSILA-S according to their biological potency. Moreover, cold NSILA-S displaces bound ¹²⁵I-labeled NSILA-S in concentrations in which it also stimulates thymidine incorporation into fibroblast DNA. Insulin displaces bound ¹²⁵I-labeled NSILA-S only at concentrations above 1 mU/ml. Glucagon, ACTH, human growth hormone and inactivated NSILA-S are ineffective. The displacement curves obtained with human serum are similar to those obtained in the presence of cold NSILA-S.