Determination of the activity of carboxypeptidase A in the blood of healthy human adults

A sensitive and highly specific assay, utilizing N-acetyl-Phe-3-thiaPhe as substrate, has been designed to make possible the direct determination of carboxypeptidase A in human serum or plasma. Measurement of the enzyme's activity in the serum of 108 blood donors has established the basal concentration for healthy human adults to be 0.068+/-0.028 U/l (x+/-1 S.D.). This is equivalent to 0.34 mu g/l of carboxypeptidase A. Such an extremely low baseline provides for a substantial dynamic range over which to assess pancreatic pathology. Previous claims in the literature for an 11 fold higher baseline have to be reexamined in view of the failure of the investigators to take into account the ability of the proenzyme form of carboxypeptidase A, which does occur in serum, to attack their substrates. (C) 1999 Elsevier Science B.V. All rights reserved.