Degradation of oxytocin by the human placenta: effect of selective inhibitors

The hydrolysis of oxytocin by human placental subcellular fractions was studied in the presence of selective inhibitors by measuring liberated amino acids by high performance liquid chromatography (HPLC). Oxytocin degradation by microsomal and lysosomal fractions was inhibited by bestatin, amastatin and puromycin. The IC50 values of these inhibitors on oxytocin degradation by both fractions were similar to those of these inhibitors on the human placental aminopeptidase M measured by L-Leu-p-nitroanilide as a substrate (LAP activity), which we reported previously. However, purified aminopeptidase M from human placental microsomal fractions could not liberate any amino acid from oxytocin. Since phosphoramidon (1 mumol/l), a putative metalloendopeptidase inhibitor, and N-benzylcarbonyl-valyl-prolinal (Z-Val-prolinal) (14 mumol/l), a selective inhibitor of post-proline endopeptidase, could not significantly influence the degradation of oxytocin by either subcellular fractions, neither enzyme seems to be actively involved in oxytocin degradation. These results strongly suggested the existence of oxytocinase(s) other than the above three enzymes in microsomal and/or lysosomal fractions of human placenta.