Fructosebisphosphatase of Plant Roots

Abstract

Summary A specific fructosebisphosphatase has been identified in pea and maize roots. It was separated from a non-specific phosphatase and partially purified using ammonium sulphate fractionation and chromatography on DEAE cellulose. The root fructosebisphosphatase appeared to be located in the cytosol. Enzyme activity was Mg 2+ -dependent (the Km for Mg 2+ being 0.15 mM), and was observed at all pH values between 6.5 and 8.5. The concentration of fructose-1,6-P 2 required for half maximal activity ranged from 0.06 mM to 0.36 mM, depending on the pH and level of Pi; it was increased when either the Pi concentration was raised from 0.14mM to 5 mM, or when the pH was lowered from 8.0 to 7.1. Unlike most fructosebisphosphatases, the root enzyme was not affected by AMP. However, Pi stimulated enzyme activity, particularly at higher pH and with higher fructose-1,6-P 2 concentrations, and preliminary evidence was obtained for strong inhibition by fructose-6-P. The physiological role of fructosebisphosphatase in roots is not clear, but the possibility that it functions in cytosolic gluconeogenesis during periods of anaerobiosis, or during conversion of amyloplast starch to sucrose, is considered.