Insulin binding and degradation in liver of fed and fasted rats: Effect of antiserum to insulin-degrading enzyme on insulin binding and degradation.

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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 72 h 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 .apprx. 10% of the activity by the cytosol fraction and showed no significant difference between fed and fasted rats. 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 .times. 10-7 M), while the maximal velocity (Vmax) 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. 125I-insulin binding to plasma membranes significantly increased in the presence of the antiserum (121 .+-. 9% of control, P < 0.05). The number of insulin receptors in the plasma membrane apparently increases, whereas the content of IDE 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.