Acid phosphatases from beet root (Beta vulgaris) plasma membranes

Several acid phosphatases (EC 3.1.3.2) were found in beet root ( Beta vulgaris L.) plasma membranes. Two of them were partially purified by an extraction of plasma membranes with octylglucoside and successive gel-filtration and anion-exchange chromatographies. With p -nitrophenyl-phosphate (pNPP) as substrate, most of the phosphatase activity was found in a fraction containing an 82-kDa protein. This phosphatase showed an optimum pH of 5.4 and was inhibited by Cu²⁺, Zn²⁺, molybdate or vanadate. The other phosphatase had a lower specific activity with p NPP, but was able to dephosphorylate phospho-myelin basic protein (phospho-MBP). This phosphatase presented two polypeptides with molecular masses of 36 and 65 kDa and was 83% inhibited by 2 n M okadaic acid, which suggests it is a PP2A protein phosphatase. As the phosphatase activity was high in soluble (non-membrane) fractions, the possibility that phosphatases in plasma membranes were soluble contaminants was assessed. Following the method of Bérczi and Møller (Plant Physiol. 116:1029, 1998), it was found that about 45% of both acid and protein phosphatase activities could be due to soluble enzymes trapped inside membrane vesicles.