Glycoprotein exhibiting immunological and enzymatic activities of human prostatic acid phosphatase

A glycoprotein (GP) which is immunochemically and biologically related to human prostatic acid phosphatase (PAP) has been isolated from human seminal plasma by ammonium sulfate precipitation followed by sequential concanavalin A:Sepharose 4B column, anion-exchange chromatography and gel filtration. The purified GP was shown to be homogeneous by disc- and sodium dodecyl sulfate: polyacrylamide gel electrophoresis. The apparent molecular weight of purified GP was estimated to be 50,000 by gel filtration and 45,000 by sodium dodecyl sulfate: gel electrophoresis. In gel diffusion against antiserum to purified PAP, a partial immunological identity was shown between GP and PAP. This was further confirmed by an inhibition reaction between GP and antiserum to purified GP by PAP. Significantly, 30% of PAP enzyme activity was inhibited by anti-GP antiserum, while only 5% was inhibited by anti-PAP antiserum. Purified GP was shown to exhibit a weak, but significant, acid phosphatase activity by hydrolyzing alpha-naphthyl phosphate at pH 5.6. The Km and Vmax for GP are 1.6 X 10(-4) M and 0.056 mumol/min/microgram protein, respectively, using alpha-naphthyl phosphate as the substrate. In the presence of anti-PAP antibody, the enzyme activity of GP was enhanced severalfold. Furthermore, the acid phosphatase activity of GP also was inhibited by tartrate, which is the most commonly used inhibitor for PAP. GP and PAP were found to have different carbohydrate content, amino acid composition, amino-terminal sequence, and peptide map. Thus, GP represents a newly identified protein. The significance of these results at molecular and clinical levels is discussed.