Ultrastructural localization of aminopeptidase A/angiotensinase and placental leucine aminopeptidase/oxytocinase in chorionic villi of human placenta.

Aims: Membrane-bound aminopeptidases in human placenta are thought to be involved in maintaining homeostasis during pregnancy by metabolizing bioactive peptides such as oxytocin and angiotensin at the interface between the fetus and mother. Because determining the precise localization of these enzymes is required to support this notion, we investigated the ultrastructural localization of two principal enzymes, aminopeptidase A (APA; EC 3.4.11.7)/angiotensinase and placental leucine aminopeptidase (P-LAP; EC 3.4.11.3)/oxytocinase in human first trimester and full-term placenta. Methods: Immunohistochemical analysis using anti-P-LAP and anti-APA antibodies was performed on ultrathin frozen sections of fixed human placental villi. Results: Transmission immunoelectron microscopy revealed that both enzymes were expressed on the surface of apical microvilli of syncytiotrophoblast cells and, to a lesser extent, on the basal infoldings. The location of the two enzymes did not vary between the first trimester and full-term placenta sections, while the staining intensities were slightly enhanced in full-term villi. Conclusions: Our observation that P-LAP and APA are present on the microvilli, which is a site of interaction between the mother and fetus, suggests possible involvement of these enzymes in cleaving peptide hormones from the fetus and mother in order to regulate bioactivity.