Characterization of a serum factor interfering with the radioimmunoassay of prostatic acid phosphatase

Storage of serum at temperatures above zero for short periods results in the spurious formation of a factor interfering with the radioimmunoassay of prostatic acid phosphatase. This factor acts by inhibiting the binding of radiolabelled tracer to the specific primary antiserum. The inhibiting factor is a protein of molecular mass 40 000 which is derived from an inactive high molecular mass precursor. Formation of the inhibiting factor is apparently reversible and its action can be neutralized by adding fresh serum to the incubation mixture. Serine-protease inhibitors of synthetic or natural origin did not influence the activity of the inhibitor. Since incubations for long periods at temperatures above freezing are standard procedures in radioimmunoassays, formation of this inhibitor may occur in susceptible sera. It is suggested that problems with establishing specific assays for prostatic acid phosphatase may be explained by the generation of this inhibiting factor.