Cellular localization of insulin-degrading enzyme in rat liver using monoclonal antibodies specific for this enzyme

Although insulin-degrading enzyme (IDE) has been implicated in the intracellular degradation of insulin, the cellular localization of this enzyme is still controversial. In the present study, we have examined the cellular localization of IDE in the rat liver by three different techniques using monoclonal antibodies. First, direct immunohistochemical staining of rat liver with one of the monoclonal antibodies revealed that IDE immunoreactivity mainly exists in parenchymal cells, especially in the vicinity of the portal tract and also in the epithelium of the bile duct under light microscopy. In the electron microscopic study, IDE immunoreactivity was found in the cytoplasm near the rough endoplasmic reticulum but not in the plasma membrane, nucleus, or mitochondria. Second, immunoblotting analysis of the subcellular fraction in rat liver showed that the monoclonal antibody specifically reacted with a single polypeptide in the cytosolic fraction, of apparent Mr 110,000, which was consistent with the Mr of IDE. However, a polypeptide band corresponding to IDE could not be observed in the plasma membrane, mitochondrial, or lysosomal fraction. Third, IDE was only detectable in the cytosolic fraction by sandwich radioimmunoassay using two monoclonal antibodies. These results all suggest that IDE is a cytosolic enzyme.