Subcellular localization and partial characterization of insulin proteolytic activity in rat liver

Using (A14-125I)-insulin as a tracer, insulin proteolytic activity in rat liver was found to be localized both to the cytosol and the endoplasmic reticulum. The membrane-associated activity was highly latent (70-80%). Both cytosolic and particulate activities had similar Km values and Mr of approx. 300 000 by gel filtration. Both were strongly inhibited by diamide (90%), but were unaffected by leupeptin or pepstatin. A comparison of the subcellular distributions with various 125I-isomers of insulin as tracers showed that both particulate and cytosolic activities were highest with (A14-125I)-insulin.