LOCALIZATION AND BIOSYNTHESIS OF AMINOPEPTIDASE-N IN PIG FETAL SMALL-INTESTINE

Background & Aims: Little is known about the expression of brush borde r enzymes in fetal enterocytes. The aim of this study was to describe the localization and biosynthesis of porcine fetal aminopeptidase N. M ethods: This study was performed using histochemistry and immunoelectr on microscopy and [S-35]methionine labeling of cultured mucosal explan ts. Results: Enzyme activity was present in the brush border membrane and extended into the apical cytoplasm. The protein was colocalized wi th cationized ferritin at the surface of endocytic structures includin g coated pits, vesicles, tubules, and large vacuoles in the apical cyt oplasm. The transient high mannose-glycosylated form of fetal aminopep tidase N was processed to the mature complex-glycosylated form at a ma rkedly slower rate than the enzyme in adult intestine. Likewise, dimer ization occurred slowly compared with the adult form of aminopeptidase N, and it took place mainly after the Golgi-associated complex glycos ylation. The enzyme had a biphasic appearance in the Mg2+-precipitated and microvillar fractions, indicating that the bulk of newly made ami nopeptidase N is transported to the brush border membrane before appea ring in the apical endocytic structures. Conclusions: In comparison wi th the adult enzyme, fetal aminopeptidase N has a more widespread subc ellular distribution with substantial amounts present in apical endocy tic compartments characteristic of the fetal enterocyte.