Two-dimensional gel electrophoresis detects prostate-specific antigen–α1-antichymotrypsin complex in serum but not in prostatic fluid

We investigated the interaction between prostate-specific antigen (PSA) and 1-antichymotrypsin (ACT) in prostatic secretions, identifying PSA and ACT in human serum, prostatic fluid, and seminal plasma by two-dimensional gel electrophoresis (2-D PAGE). Both PSA and ACT were detected in all three body fluids, but PSA-ACT complex was detected only in serum. Moreover, the 2-D PAGE Western blot staining profile for ACT from serum differed from that for prostatic fluid or seminal plasma. Incubation of prostatic fluid with purified ACT led to formation of PSA-ACT complex. Incubation of prostatic fluid with purified PSA, however, failed to form the complex, suggesting that the ACT in prostatic fluid was inactive or inhibited. Given that physiological concentrations of zinc inhibited the formation of PSA-ACT complex, we consider zinc a possible physiological inhibitor of the formation of the PSA-ACT complex. These results indicate that the failure to detect the PSA-ACT complex in prostatic fluid could be related to the inactivation of ACT, the presence of inhibitors (e.g., zinc), or simply the PSA:ACT ratio in the fluid.