Sedimentation characteristics of subcellular vesicles associated with internalized insulin and those bound with...

Comparative studies were made on the sedimentation characteristics of microsomal vesicles associated with internalized [125I]iodoinsulin and those bound with intracellular glucose transport activity. Upon linear sucrose density gradient centrifugation, the internalized hormone formed a peak slightly, but significantly, on the higher density side of the peak of intracellular glucose transport activity. After a long centrifugation, the peak of 125I activity became lower and broader than that of glucose transport activity. Internalized 125I activity was also found in the medium-density microsomal fraction, which had little glucose transport activity. Accumulation of 125I activity in the medium-density fraction and that in the low-density fraction were both completed in approximately 10 min. Under basal conditions, little, if any, insulin binding activity was detectable in either the medium- or low-density microsomal fractions; in contrast, some glucose transport activity was always present in the low-density fraction. These results indicate that the subcellular distribution of internalized insulin and of intracellular glucose transport activity are different, suggesting that the pathways of intracellular processing of the insulin receptor and the glucose transport mechanism are different.