Localization and biosynthesis of aminopeptidase N in pig fetal small intestine

Abstract

Background & Aims : Little is known about the expression of brush border enzymes in fetal enterocytes. The aim of this study was to describe the localization and biosynthesis of porcine fetal aminopeptidase N. Methods : This study was performed using histochemistry and immunoelectron microscopy and [ 35S]methionine labeling of cultured mucosal explants. Results : Enzyme activity was present in the brush border membrane and extended into the apical cytoplasm. The protein was colocalized with cationized ferritin at the surface of endocytic structures including coated pits, vesicles, tubules, and large vacuoles in the apical cytoplasm. The transient high mannose-glycosylated form of fetal aminopeptidase N was processed to the mature complex-glycosylated form at a markedly slower rate than the enzyme in adult intestine. Likewise, dimerization occurred slowly compared with the adult form of aminopeptidase N, and it took place mainly after the Golgi-associated complex glycosylation. The enzyme had a biphasic appearance in the Mg 2+-precipitated and microvillar fractions, indicating that the bulk of newly made aminopeptidase N is transported to the brush border membrane before appearing in the apical endocytic structures. Conclusions : In comparison with the adult enzyme, fetal aminopeptidase N has a more widespread subcellular distribution with substantial amounts present in apical endocytic compartments characteristic of the fetal enterocyte.