Insulin Binding and Degradation in Liver of Fed and Fasted Rats

Effect of Antiserum to Insulin-degrading Enzyme on Insulin Binding and Degradation

Abstract

Kinetic studies on ¹²⁵I-insulin binding and degradation were examined using liver plasma membrane and cytosol fractions from fed and fasted rats.
Liver plasma membranes of 72h fasted rats bound twice as much insulin as did membranes of fed rats. Scatchard analysis showed that the enhancement of ¹²⁵I-insulin binding in fasted rats was due to an increase of insulin receptor concentrations in the membranes. Specific activity of insulin degradation by plasma membranes was about 10% of the activity by the cytosol fraction and showed no significant difference between fed and fasted rats. On the other hand, insulin-degrading activity in the cytosol fraction from fasted rats was significantly lower than fed rats (P<0.02). Quantitative analysis of insulin degradation in the cytosol fraction revealed a similar Km for both groups (1.8 to 2.5×10^-7M), while the maximal velocity (V_max) in fasted rats was significantly lower than that of fed rats (P<0.02).
The antiserum to insulin-degrading enzyme (IDE) purified from pig skeletal muscle was able to remove insulin-degrading activity in both the cytosol (34±7 vs 79±3% of the control, P<0.001) and plasma membrane fractions (49±7 vs 81±6% of control, P<0.01) by immunoprecipitation when compared with normal rabbit serum. Furthermore, ¹²⁵Iinsulin binding to plasma membranes significantly increased in the presence of the antiserum (121±9% of control, P<0.05).
These data suggest that the number of insulin receptors in the plasma membrane increases, whereas the content of insulin-degrading enzyme (s) in the cytosol fraction decreases in the hypoinsulinemic state. Since insulin-degrading activity in plasma membranes does not change in the fasted state, insulin seems to be mainly degraded by intracellular enzyme (s) including IDE rather than the insulin-degrading system of plasma membranes in the rat liver.