Phosphotyrosine phosphatase activity of human and canine acid phosphatases of prostatic origin

Human and canine prostatic specimens containing high levels of acid phosphatase (AP) activity were tested, at acid pH, for their ability to hydrolyze the major phosphoaminoacids present in phosphorylated proteins, phosphoserine (p-ser), phosphothreonine (p-thr), and phosphotyrosine (p-tyr). The cleavage of a synthetic substrate, para-nitrophenyl-phosphate (p-npp), was also measured as an indicator of AP activity; its inhibition by sodium-L-tartrate (T) was used as a criterion to identify prostatic acid phosphatase (PAP). It was found that: 1) the Km of p-tyr and p-npp were 2.0 mM and 0.41 mM, respectively, with similar Vmax values (0.078 and 0.087 mumoles of phosphate (Pi) liberated per minute per milligram of protein); 2) the ID50 were 0.25 mM and 0.50 mM with sodium orthovanadate (VO4) and T, respectively, using p-npp as substrate-with p-tyr as substrate, the values obtained were 0.016 mM and 0.11 mM, respectively; 3) activity toward p-ser and p-thr was minimal; 4) native PAP from dog seminal plasma, with a molecular weight of 90-100 kD, as determined by gel filtration on HPLC, hydrolyzed p-tyr preferentially, and this phosphatase (Pase) activity was also strongly inhibited by both T and VO4; and 5) the AP present in human and canine prostatic tissue and cells, as well as in their secretions, also preferentially hydrolyzed phosphotyrosine, and it was inhibited by T and VO4. It is proposed that these p-tyr Pases may be involved in the local regulation of prostatic growth.