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.