Isolation and partial characterization of a water-soluble receptor protein of insulin

By using the chelate EDTA at low concentration to remove the bivalent cations in the plasma membranes and followed by n-butanol to extract the membrane lipids, we have obtained a water-soluble insulin receptor without detergent from the liver-cell plasma membranes. This receptor protein does not precipitate by centrifugation at 300,000 X g for 70 min, nor does it retain on 0.22 micron millipore filter. It does not retard on Sephadex G-200 gel chromatographic column either. It is thus proved that the insulin-receptor protein obtained by this method is completely soluble in water. The dissociation constant of the water-soluble receptor of insulin at 24 degrees C is calculated to be 3.6 X 10(-9) M and its isoelectric point was approximately pH 4.1. As the fluorescence hydrophobic probe 1,8-ANS does not significantly affect the binding activity between the receptor and the insulin, it seems that in addition to a hydrophobic binding mechanism, there may exist some other forces of interaction.