Immunohistochemical evaluation of endomannosidase distribution in rat tissues: evidence for cell type-specific expression

Asparagine-linked oligosaccharides of glycoproteins are subject to a series of trimming reactions by glucosidases and mannosidases in the endoplasmic reticulum which result in the removal of all three glucose residues and sev eral of the nine mannose residues. At present, endomannosidase represents t he only processing enzyme which cleaves internally and provides an alternat e deglucosylation pathway. However, in contrast to the endoplasmic reticulu m residential proteins glucosidase I and II, endomannosidase is primarily s ituated in the Golgi apparatus of rat liver hepatocytes and hepatocyte cell lines. We have performed a confocal immunohistochemical study to investiga te endomannosidase in various rat tissues and used a monoclonal antibody ag ainst Golgi mannosidase II as a marker for the Golgi apparatus. Although im munofluorescence for both endomannosidase and Golgi mannosidase II was dete ctable in the epithelia of many tissues, renal proximal tubular cells, cort ex and medulla of adrenal gland, gastric mucosa, and Leydig cells of testis were unreactive for endomannosidase. Furthermore, the endothelia in all st udied tissues were unreactive for endomannosidase but positive for Golgi ma nnosidase II. It is concluded that by immunohistochemistry endomannosidase exhibits a cell type-specific expression in rat tissues.