Arginine carboxypeptidase (CPR) in human plasma determined with sandwich ELISA

There are two types of carboxypeptidases present in human blood, carboxypeptidase N (CPN) and arginine carboxypeptidase (CPR). CPR is generated during coagulation from a precursor (proCPR) which can be converted to the active form by trypsin in vitro. Since it is difficult to distinguish the two types of carboxypeptidases in human blood by the measurement of enzyme activity, we established a quantitative sandwich ELISA by which CPR can be quantitated. The amount of CPR in plasma, fresh serum and heated serum were essentially the same. Therefore the ELISA assay does not distinguish proCPR, activated CPR and inactivated CPR. With the ELISA method, CPR was quantitated in plasma from fifty patients with rheumatoid arthritis and eleven patients with severe hepatitis as well as healthy individuals. The amount of CPR in plasma obtained from patients with rheumatoid arthritis was not found to be lower than that of normal subjects. Furthermore, the patients who suffered severe hepatitis and had very low levels of CPR-total were fatal. This suggests that a decrease of CPR level might be a good indication of a patient's prognosis to death by hepatitis.