Identification of an alanine aminopeptidase in human maternal serum as a membrane-bound aminopeptidase N

In addition to cystine Aminopeptidase (oxytocinase) alanine Aminopeptidase is present at high levels in the serum of pregnant women. In this study we compared the Enzyme with membrane-bound Aminopeptidase N purified from human placenta. Comparison of catalytic and immunological properties and N-terminal sequence analyses revealed that the Enzymes were differentially processed derivatives of the same protein, and that the N-terminal 68 residues of Aminopeptidase N were deleted in the alanine Aminopeptidase. The deleted sequence contains a small cytoplasmic region, a hydrophobic transmembrane domain and a junctional domain. These results suggest that the Enzyme may be released into the maternal circulation as a result of lacking these three domains.