Macro-prostatic acid phosphatase in a patient's serum.

A patient without prostatic carcinoma had a high concentration of prostatic acid phosphatase (PAP; EC 3.1.3.2) in his serum. This PAP was bound to IgG ("macro-PAP"), and IgG autoantibodies against PAP were demonstrated in serum. The patient's IgG prolonged the biological half-life of radiolabeled PAP in rats, suggesting that the formation of IgG-PAP complexes was responsible for decreased PAP catabolism. Furthermore, macro-PAP was inactivated in serum more slowly than PAP. These factors accounted for the increases in the enzymatic activity and antigenic concentration of PAP measured in the patient's serum. Inappropriate therapy was prescribed on the basis of this laboratory result. The diagnosis of prostatic carcinoma requires clinical or histological evidence of malignant disease, and should not rely solely on PAP measurements.