Purification and characterization of multiple forms of soybean seed acid phosphatases

Four isoforms of acid phosphatase (EC 3.1.3.2), AP1, AP2, AP3A and AP3B, have been detected and partially purified from soybean seed (Glycine max) through DEAE- and SP-Sephadex chromatographies. Specific activity values of 822, 163, 14 and 66 nkat·mg−1 were obtained for AP1 (903-fold purification), AP2 (180-fold), AP3A (15-fold), and AP3B (73-fold), respectively, using p-nitrophenylphosphate as substrate. Relative native molecular mass values for AP1. AP2, AP3A and AP3B, determined by gel filtration on calibrated SW-300 Waters Protein Glass column, were found to be 51 000, 58 000, 52 000 and 30 000, respectively. All four acid phosphatase isoforms presented a carbohydrate moiety in their structures and revealed only single phosphatase activity bands following nondenaturing polyacrylamide gel electrophoresis, at pH 8.3. AP1 and AP2 exhibited greater substrate specificity than AP3A and AP3B. The Km values were determined for p-nitrophenylphosphate, tyrosinephosphate and inorganic pyrophosphate, at pH 5.0 and 37 °C. The acid phosphatases presented the following apparent Km values: AP1 (pNPP — 0.49, PPi — 0.21 and TyrP — 1.14 mM); AP2 (pNPP — 0.38, PPi — 1.33 and TyrP — 1.14 mM); AP3A (pNPP -0.20, PPi — 0.16 and TyrP — 0.19 mM) and AP3B (pNPP — 0.086, PPi — 0.17 and TyrP — 0.17 mM). All four isoforms were inhibited by inorganic phosphate, fluoride, vanadate, molybdate, Cu2+ and Zn2+. The soybean seed acid phosphatases did not catalyze the transphosphorylation reaction since no stimulation was observed with inorganic phosphate acceptors, such as glycerol, methanol and ethanol.