Medicolegal identification of seminal fluid by the radioimmunoassay of prostatic acid phosphatase

For the purpose of applying a radioimmunoassay (RIA) of prostatic acid phosphatase (PAP) for the medicolegal identification of seminal fluids and seminal stains, the usefulness of this method has been investigated for a comparison with the conventional method of an enzyme assay (EA). The detection limit of the PAP level was found to be about 0.5 ng/ml by the RIA method, while that of the PAP activity was about 0.2 King-Armstrong units (K.A.U.) by the EA method. The mean concentration and activity of PAP in the seminal fluid were 1,450,690 ng/ml and 563,290 K.A.U. respectively, these values being markedly higher than those of other human body fluids and secreta. The PAP contents and activities in the seminal fluids from normal, oligospermic and azoospermic human subjects were not significantly (p<0.05) different among these groups. The presence of a PAP-antigenic substance in juices from various vegetables and fruits was not detected by the RIA method, while pseudo-PAP activities were detected in all such samples tested by the EA method, indicating the superior specificity of the RIA method. Both the antigenicity and the enzyme activity of PAP remained stable at -80°C or -20°C. At 4°C, however, while the antigenicity still was stable, the enzyme activity decreased to 65.0% after 6 months. In heated seminal fluid samples, both the antigenicity and the enzyme activity of PAP were relatively unstable, and the PAP enzyme activity and content were reduced to 3.5% and 13.6% respectively after 1 min at 60°C. When seminal stains were heated for 30 min, both the antigenicity and enzyme activity of PAP were relatively stable in a temperature range from 50°C to 150°C. At 200°C, both values decreased by about 5%, and the PAP enzyme activity could not be detected after 30 min at 250°C. In the seminal fluid samples stored at room temperature, the antigenicity of PAP was stable for 6 months and declined to 20.3% after 9 months, whereas the enzyme acativity of PAP was reduced to 30.7% after 6 months and nearly disappeared after 9 months. In the seminal stains stored at room temperature, both the antigenicity and enzyme activity of PAP remained relatively stable, their residual percentages after 12 months being 34.4% and 28.4%, respectively. As a result of an attempt to detect PAP in old seminal stains stored at room temperature, fairly high PAP antigenicity and enzyme activity were detected from seminal stains dating back to 10 years, making it still possible to evidence the presence of seminal fluid. The presence and absence of seminal fluid in 6 specimens of vaginal swabs collected from medicolegal autopsies were examined by the RIA and EA methods, and no disagreement was observed in the result of either methods. One of 2 RIA-positive vaginal swabs was obtained from a woman buried for 19 days. In this case, no sperm was detected by microscopic examination, and the result of the SM test was weakly positive. These results clearly indicate that the present method for detecting seminal fluid utilizing the microquantitative assay of PAP by RIA is superior to the conventional enzyme activity measurement method in both specificity and sensitivity.