Purification and Characterization of a New Human Prostatic Acid Phosphatase Isoenzyme

A new enzyme has been purified to homogeneity from human seminal plasma as determined by Polyacrylamide gel electrophoresis. This enzyme is shown to be an acid phosphatase (EC 3.1.3.2) by hydrolyzing a variety of phosphomonoesters at acidic pH, and hence designated as prostatic acid phosphatase II (PAP-II) to be differentiated from the conventional prostatic acid phosphatase (PAP) designated as PAP-I, which is isolated from the same source. PAP-II has a molecular weight (M_r) of 120 000 as estimated by gel filtration and possesses two subunits of M_r 55 000 each as revealed by sodium dodecyl sulfate-acrylamide gel electrophoresis, in comparison with molecular weights of 100 000 and 50 000, respectively, for PAP-I. The purified PAP-II demonstrates multiple pIs at 4.70–4.90, while those of PAP-I are at 4.84–5.33. PAP-II is inhibited by Fe³⁺, Ca²⁺, and La³⁺, whereas PAP-I is not affected at all. In addition to seminal plasma, PAP-II is detected only in tissue extracts of carcinoma prostate, benign prostate hypertrophy and normal prostate. Upon Immunoelectrophoresis, PAP-II shows a narrower precipitin arc with goat anti-PAP-I antiserum in reference to PAP-I, although an identical reactivity is detected by immunodiffusion. An immunological identity between PAP-I and PAP-II also is shown by their reactions with rabbit anti-PAP-II antiserum in immunodiffusion. Apart from these immunologic characteristics, results obtained from thermostability experiments, carbohydrate determination, in vivo clearance rate study, amino acid composition analysis, and peptide mapping data indicate that PAP-II is different from PAP-I. The vast difference in amino acid compositions rules out the possibilities that PAP-II and PAP-I are merely two distinctly different classes of glycosylated molecules and that PAP-II and PAP-I are the products of the same gene. It is concluded that PAP-II represents a new human PAP isoenzyme.