Time-course of Insulin Degradation in Perifused Isolated Rat Adipose Cells

Isolated fat cells from rat epididymal adipose tissue were preincubated with 50 microU/ml (0.33 nM) 125I-insulin at 23 degrees C to enhance binding while retarding degradation. The fat cells were then perifused at that temperature to remove unbound 125I-insulin, and fractions of perifusate were collected each minute. The temperature of the cells in the perifusion chamber was then rapidly increased to 37 degrees C, and perifusion was continued. The fat cells degraded a portion of the bound 125I-insulin measured by loss of immunoprecipitability with excess antisera to insulin. The percentage of degraded 125I-insulin dissociating from the fat cells increased progressively with time at 37 degrees C, and the rateof dissociation of 125I-insulin degradation products showed a first-order dependence on the amount of degraded 125I-insulin bound to the cells. To explain this first-order dependence it is necessary to postulate a "processing" step after binding and before degradation. The first-order rate constant at 37 degrees C is 0.023 +/- 0.004 min-1. Fast and slow dissociating components can be resolved from kinetic plots of the dissociation of undegraded 125I-insulin (immunoprecipitable) from the isolated fat cells. The antilipolytic activity of the 125I-insulin on epinephrine-stimulated lipolysis is evident over much of the time-course of dissociation. A model for the degradation of insulin bound to isolated fat cells is discussed.