The stability of prostatic acid phosphatase, as measured by a capture immunoenzyme assay

Abstract

A capture immunoenzyme assay (CIEA) for prostatic acid phosphatase (PAP) was developed and used to study the stability of this isoenzyme. Immunospecifically purified goat antibodies to PAP were covalently bound to special discs and used to capture the enzyme in serum samples in a weakly acidic medium during the first incubation (2 h) at 37°C. The capture enzyme was then measured by its catalytic activity with p-nitrophenyl phosphate as substrate during the second incubation (1 h) at 37°C. As much as 98% of the PAP in test specimens was captured and measured by this CIEA. The test results were expressed as enzymatic activity (U/l), extrapolated from a standard curve which was linear between 0.026 and 70 U/l. In test sera stored at 4°C, the PAP was variably stable for 7 to 70 days, but the enzyme was quite stable in serum when stored at −20°C for at least 156 days. At room temperature, when the sera were appropriately acidified, there was no loss of enzymatic activity for periods of 15 days, and in some cases, a large proportion of activity was still intact after 70 days. At 4°C, as well as − 20°C, acidified serum and the partially purified PAP standard showed complete stability for at least 7 months. The CIEA reactivity of positive test specimens was inhibited by l( + )-tartaric acid, but not by cupric sulfate. The acid phosphatases of blood cell extracts were non-reactive in the CIEA procedure. The CIEA results of 224 serum samples from patients with and without prostate cancer correlated very well with those obtained by two direct enzymatic and two commercial RIA procedures, with correlation coefficients between 0.960 and 0.993, and diagnostic agreement between 86% and 100%.