Prostate-specific antigen triggers transformation of seminal α2-macroglobulin (α2-M) and its binding to α2-macroglobulin receptor low-density lipoprotein receptor-related protein (α2-M-R/LRP) on human spermatozoa

BACKGROUND. The aim of this study was to identify the proteolytic activity which triggers the transformation of human alpha 2-macroglobulin (alpha 2-M) in seminal fluid and its binding to its receptor. METHODS. Measurement of the concentrations of total and transformed alpha 2-M in seminal fluid was accomplished by ELISA. Zymography of seminal plasma was performed in SDS-polyacrylamide gels containing casein as proteolytic substrate. Rate electrophoresis, SDS-PAGE, and Western blotting were applied to study the complex formation of prostate-specific antigen (PSA) with alpha 2-M. Ligand-binding analysis of sperm cells was performed using [I-125]-labeled proteins. Detection of receptor on sperm cells was achieved by immunofluorescence. RESULTS. The mean concentration of total a2-M in a random collection of seminal plasma was 4.6 mu g/ml. On average, between 33-98% of the inhibitor was found to be transformed. Zymography of seminal plasma revealed a proteolytic activity which is associated with a 33-kDa protein identified as PSA. Its proteolytic activity could be inhibited by alpha 2-M. Both purified PSA and seminal plasma were capable of transforming native alpha 2-M. Binding of PSA to alpha 2-M triggers the exposition of receptor binding sites in the inhibitor molecule, which causes binding of the complex to alpha 2-M-R/LRP identified on spermatozoa. CONCLUSIONS. PSA, the main proteinase in seminal fluid, is responsible for the transformation of alpha 2-M and for its binding to a2-M-R/LRP present on spermatozoa. The binding of alpha 2-M-PSA complexes to the spermatozoa receptor may exert an impact on normal sperm-cell functions. Prostate 36:219-225, 1998. (C) 1998 Wiley-Liss, Inc.