Immunologic characterization of human seminal leucine aminopeptidase (LAP) and its medicolegal use.

A new method for identification of seminal stains is described, based on the immunologic demonstration of leucine aminopeptidase (LAP), which is extremely abundant in human semen and specific for the prostate as well as semen. An antiserum against human seminal plasma was obtained by repeated immunization of rabbits with seminal plasma and Freund's adjuvant. Ouchterlony's double immunodiffusion test and Culliford's precipitin electrophoresis were performed to demonstrate specific proteins of seminal plasma. LAP activity was visualized with L-leucyl-beta-naphthylamide as substrate and with Fast Garnet GBC as coupler. The immunologic analysis of LAP produced two precipitin lines with enzyme activity. One was observed in kidney, jejunum, pancreas, prostate, as well as in semen, and was completely absorbed with kidney homogenates. The other was found only in semen and the prostate and was not absorbed with kidney homogenates. When the anti-seminal plasma serum absorbed with the kidney was used, the semen-specific LAP could be demonstrated by precipitin electrophoresis only in seminal stains stored for up to 2 months, whereas it was not demonstrated in stains from other human body fluids. By means of precipitin electrophoresis the detection of the semen-specific LAP was possible at semen dilutions of up to 1:32. The method described here greatly enhances the value of semen identification and is quite recommendable for the examination of stains in medico-legal practice.