Stability of immunologic activity of human prostatic acid phosphatase in serum

The stability of the immunologic activity of human serum prostatic acid phosphatase (iPAP;EC 3.1.3.2) under various storage conditions was evaluated. Although more stable than the enzymic activity of serum PAP, serum iPAP was inactivated rapidly at room temperature at neutral and especially at alkaline pH. Purified iPAP in a phosphate buffer (pH 7.5) was stable for at least three days at room temperature but showed instability similar to that of serum iPAP when added to heat-treated serum (30 min at 56 degrees C, pH 8.3). Addition of a protease inhibitor, a sulfhydryl agent, or a chelating agent failed to preserve iPAP. Acidification of serum with an acetate buffer (5 mol/L, pH 5.0), 20 microliters/mL of serum, protected but could not restore serum iPAP activity. When stored at 24 degrees C, serum iPAP was most stable at pH 6.5. Acidification had little effect on PAP values by radioimmunoassay, iPAP values from acidified and unacidified serum samples being essentially the same. We recommend that blood samples drawn for PAP immunoassays be kept at 4 degrees C and that serum samples be separated and acidified as soon as practical. For long-term storage, acidified serum samples should be kept in small aliquots at -20 or -70 degrees C.