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Journal of Clinical Endocrinology & Metabolism, Vol 74, 1116-1121, Copyright © 1992 by Endocrine Society
ARTICLES |
V Pezzino, V Papa, A Costantino, L Frittitta, P Russo, ID Goldfine and R Vigneri
Cattedra di Scienza delle Costituzioni, Universita di Catania, Italy.
With a two-step purification procedure employing sequential affinity chromatography with insulin receptor monoclonal antibody followed by wheat germ agglutinin, we isolated from the plasma of two healthy individuals a material that reacted in a specific RIA for insulin receptors. This material produced dilution curves that were parallel to a human placental insulin receptor standard. This material also bound [125I]insulin; competition-inhibition curves revealed an ED50 of 0.3 nM, a value similar to that obtained with placental insulin receptors. The material purified from plasma was then labeled with [125I] Bolton- Hunter reagent, followed by polyacrylamide gel electrophoresis under reducing conditions and autoradiography. A band at 135 kilodaltons (kDa) was observed, corresponding to the alpha-subunit of the insulin receptor. Several bands ranging from 82-46 kDa were also detected. One or more of these fragments had intrinsic autophosphorylation activity, but only the 82-kDa band activity was responsive to insulin. In addition, employing the synthetic substrate poly(Glu4:Tyr1), no insulin- sensitive tyrosine kinase activity was present. These studies demonstrate, therefore, that insulin receptor-derived material is present in human plasma. This material retains high affinity insulin binding, but has an altered beta-subunit that is devoid of insulin- responsive tyrosine kinase activity.
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