Effects of insulin on insulin-binding components extracted from rat fat cell membranes

Abstract

WE have previously shown the presence of two distinct insulin-binding species (designated species I and II) in a Triton X-100 extract of fat cell membranes¹. They were separable electrophoretically and displayed different insulin-binding characteristics. Scatchard analysis of the insulin-binding data for species I indicated a complex, non-classical association, illustrated by a curvilinear plot. The characteristics of this curve are quite similar to those of the binding curves obtained with unfractionated detergent extract or with intact plasma membranes¹. In contrast to these findings, insulin-binding to species II was characterised by a relatively low affinity and a linear Scatchard plot. Although these findings indicate different insulin-binding and electrophoretic properties for the two species, the possibility remains that they are related structurally. For example, the K_d values for the low-affinity portion of the peak I Scatchard plot and for insulin binding to peak II are similar. Furthermore, following the incubation of isolated peak I with ¹²⁵I-labelled insulin and subsequent electrophoresis, a small amount of radioactivity migrated in the position of peak II (ref. 1). We report here that interconversion occurs between the two binding species and that the conversion is mediated by insulin.