Characterization of three aminopeptidases purified from maternal serum

The biochemical characteristics of aminopeptidase A (EC 3.4.11.7), oxytocinase (EC 3.4.11.3) and alanyl aminopeptidase (EC 3.4.11.2) purified from serum of pregnant women were compared. Aminopeptidase A hydrolysed only acidic amino acid derivatives, whereas oxytocinase and alanyl aminopeptidase had partially overlapping broad substrate specificities. Oxytocinase showed the highest Vmax value with LeuNA but the lowest Km value with ArgNA (Km 0.059 +/- 0.08 mmol/l). Alanyl aminopeptidase hydrolysed AlaNA most rapidly, but showed the highest affinity for LysNA (Km 0.054 +/- 0.006 mmol/l). The enzymes were sensitive to EDTA. Co2+, Ni2+ and Zn2+ were able to reactivate all suppressed enzymes, but Mn2+ reactivated only aminopeptidase A after EDTA inhibition. The alkaline earth metals were activators of aminopeptidase A, while Co2+ activated only alanyl aminopeptidase. This enzyme was the most sensitive to L-amino acids. Acidic amino acids inhibited aminopeptidase A but had no effect on the two other enzymes. Oxytocinase was most sensitive to thermal treatment. Amastatin did not inhibit oxytocinase, whereas aminopeptidase A was more resistant than alanyl aminopeptidase to this effector.