Abstract
Two thylakoid-associated phosphatases from soybean ( Glycine max) have been purified and characterized. Both enzymes were prepared by ultrasonication of thylakoids, (NH 4) 2SO 4 fractionation, gel filtration, cation, and anion exchange chromatography. Phosphatase I and II copurified during the first steps of purification and were separated by ion exchange chromatography on CM cellulose. The phosphatases have a molecular mass of 70 kDa. They exhibit a pH optimum at 5.5 and 6, respectively. Mg 2+ ions are not required for activity. Several phosphorylated substrates are hydrolyzed, including phenyl and 4-nitrophenyl phosphate, 3-phosphoglycerate, and fructose 1,6-bisphosphate. The thylakoid-associated phosphatases also possess phosphoprotein phosphatase activity towards 32P-labelled casein and chloroplast LHC II as substrates. Fluoride inhibited the hydrolysis of nitrophenylphosphate and [ 32P]casein but not the protein phosphatase activity towards 32P-LHC II. The protein serine/threonine phosphatase inhibitor okadaic acid did not affect these phosphatases at all.
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