An acid phosphatase (EC 3.1.3.2) has been identified and purified from castor bean (Ricinus communis L., IAC-80) seed through sulphopropyl (SP)-Sephadex, diethylaminoethyl (DEAE)-Sephadex, Sephacryl S-200, and Concanavalin A-Sepharose chromatography. The enzyme was purified 2000-fold to homogeneity, with a final specific activity of 3.8 mu kat mg(-1) protein. The purified enzyme revealed a single diffuse band with phosphatase activity on nondenaturing polyacrylamide gel electrophoresis, at pH 8.3. The relative molecular mass, determined by high-performance liquid chromatography (HPLC), was found to be 60 kDa, The acid phosphatase had a pH optimum of 5.5 and an apparent K-m value for p-nitrophenylphosphate of 0.52 mM, The enzyme-catalyzed reaction was inhibited by inorganic phosphate, fluoride, vanadate, molybdate, p-chloromercuribenzoate (pCMB), Cu2+ and Zn2+, The strong inhibition by pCMB, Cu2+ and vanadate suggests the presence of sulfhydryl groups essential for catalysis. The castor bean enzyme also recognized tyrosine-phosphate and inorganic pyrophosphate (PPi) as substrate. The highest specificity constant (V-max/K-m) was observed with PPi, making it a potential physiological substrate.1072151158