Determination of cystyl-aminopeptidase. Isoenzymes in seminal plasma and serum of different origin.

The "oxytocinase" activity has been determined as the CAP (cystyl-aminopeptidase, EC 3.4.11.3) activity with several substrates. It was demonstrated that serum samples from pregnant women and patients with serious liver disease had increased CAP activities compared with normal serum. It was also shown that seminal plasma had very high CAP activity. The pH optimum for the enzymes from the various sources differed, as did the metal dependence. Furthermore, samples from patients with liver disease had higher CAP activities when assayed in buffers containing amino groups, in contrast to the other samples. Gel chromatography and gel electrophoresis revealed the presence of several isoenzymes. The serum from pregnant women contained an isoenzyme neither present in normal plasma nor in any of the other sources tested. It is possible that this isoenzymes represents the true oxytocinase. CAP and LAP (leucine aminopeptidase) activities were very well correlated in serum from pregnant women and seminal plasma, while the correlation for samples from patients with liver disease was not as good. When care was taken to determine the CAP activity at the right pH and with the appropriate buffer, the risk for interference from other enzymes was minimized in the determination of oxytocinase as CAP activity.