D-Glucose uptake and insulin binding by the human adipose cell plasma membrane as a function of its polypeptide composition

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Abstract
A preparation of plasma membranes isolated from human omental lipocytes is composed of about 15 major polypeptide components including 3 major glycoproteins with an apparent MW range from 100,000-23,000, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Extraction of this membrane preparation with NaI or 2,3-dimethylmaleic anhydride solubilized 50 and 70% of the membrane protein, respectively, resulting from the extensive extraction of protein from all but 2 of the major membrane polypeptide components. This removal of protein did not affect the membrane''s stereospecific D-glucose-uptake activity but did reduce its total specific [125I]insulin-binding activity by 46-67%. The binding of [125I]insulin to its specific receptor on lipocyte plasma membranes was detected at physiologic concentrations of the hormone and could be competitively displaced by increasing concentrations of native insulin. The kinetic behavior of this reaction was approximated by Scatchard analysis, and the affinity and binding capacity of the plasma membrane for insulin were increased at lower temperatures. These results suggest that D-glucose transport in human adipose tissue is mediated by an intrinsic component of the hydrophobic structure of the lipocyte plasma membrane, and represent a partial purification of this component. These studies demonstrate and characterize the binding of insulin to the plasma membrane isolated from human lipocytes. A quantitative study of this binding reaction may provide further understanding of the mechanisms underlying the decreased insulin responsiveness characteristic of human diabetes.

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