Proteolytic Activity in the Insulin Receptor

When a highly purified preparation of rat liver insulin receptor is incubated with trypsin, the receptor develops hydrolytic activity towards N alpha-benzoyl-L-arginine ethyl ester, N alpha-p-tosyl-L-arginine methyl ester, and N alpha-benzoyl-DL-arginine-p-nitroanilide, (compounds which are synthetic substrates of trypsin). The activity toward N alpha-benzoyl-DL-arginine-p-nitroanilide is inhibited by soybean trypsin inhibitor but not by N alpha-p-tosyl-L-lysil chloromethyl ketone. These data are consistent with the presence of proteolytic activity in the insulin receptor specific for the bonds whose carbonyl functions are provided by arginine but not by lysine. Furthermore we found that the esterase activity toward N alpha-benzoyl-L-arginine ethyl ester in the presence of trypsin is enhanced by insulin, and that the concentration of insulin that produced the half maximum stimulation is of the same magnitude as the dissociation constant for the soluble receptor. These data suggest that the insulin receptor is a zymogen, activated by trypsin, and based on its specific activity, may be the protease which releases a peptide chemical mediator of intracellular action of insulin.