The Insulin Degrading Activity of Isolated Fat Cell Ghosts

Utilizing the isolated fat cell ghost technique of Rodbell the insulin degrading enzyme activity of fat cell membrane was investigated. These findings locate an enzyme system which has not been previously reported since other investigators dealt with the intracellular insoluble lipid-rich fraction of the cell and with whole tissue homogenates. Some of the properties of the enzyme system were determined. The pH-activity curve is broad and bell-shaped, similar to other proleolytic [sic] enzymes, and the greatest activity was seen at pH 9. The enzyme system is temperature sensitive . A degree of specificity is indicated by the absence of an effect on glucagon, a lack of inhibition by Trasylol and the failure of gamma globulin, albumin and glucagon to show substrate competition when used in equimolar concentrations. Unlabeled insulin competed with labeled insulin for the active site or sites of the enzyme system. Addition of n-ethylmaleimide resulted in inhibition of insulin degrading activity indicating that sulfhydral [sic] groups play a role in formation of the enzyme substrate complex. L-tryptophan, which inhibits insulinase in vivo, also inhibits the insulin degrading activity of the ghost preparation. The insulin degrading activity of normal rat tissue was compared with that of obese rats and normal rabbits and humans. The obese rat showed no significant difference in activity. The rabbit had about one twentieth and the human one fourth of the activity of the normal rat