The endocytotic-internalization pathway of insulin metabolism: relationship to insulin degradation and activation of glucose transport.

Abstract

The relationship between receptor-mediated internalization of [125I]insulin and the process of intracellular degradation of insulin and activation of glucose transport was studied using isolated rat adipocytes. After an initial 5- to 10-min lag period, insulin was rapidly internalized at a linear rate for the first 30 min, but began to plateau at later times. To estimate how much insulin was internalized, chloroquine-treated cells were incubated at 16 C with [125I]insulin, washed to remove all unbound insulin, and warmed to 37 C to initiate endocytosis. Using this approach, it was calculated that between 16–32% of initially bound [125I]insulin was endocytosed. Internalized [125I]insulin in chloroquine-treated cells was subsequently released with a t½ of approximately 2.5 h. When receptor-bound [125I]insulin was the only substrate for insulin degradation, it was found that adipocytes internalized and degraded insulin, and degradation products rapidly accumulated in the buffer for the first 30 min, by which...