A COMPARATIVE STUDY OF TWO FORMS OF GLUTAMINE SYNTHETASE FROM RICE LEAVES

Two forms of glutamine synthetases designated as GSI and GSII were separated by DEAE-Sephacel chromatography from rice leaves at different growth stages. The activity of GSI could maintain a rather constant level at all growth stages, but the activity of GSII decreased markedly after tillering stage. By using 2'5'ADP-Sepharose . 4B affinity chromatography and hydroxyapatite absorption, GSI was purified to apparent homogeneity and GSII was nearly homogeneity. GSII was more heat-labile than GSI, and GSII electrophoretically migrated faster then GSI on 5?á Polyacrylamide gel at pH 8.3. Km values of GSI for NH4C1, L-glutamate and. ATP were 0.15 mM, 1.89 mM and 1.39 mM, respectively; Km values of GSII for NH4C1 and ATP were 0.75 mM and 1.72 mM, respectively. The Km value of GSII for L-glutamate showed biphasic with 6.45 mM and 4.17 mM at high and low concentrations, respectively. In the presence of Mg2+, the pH optima for GSI and GSII were 7.1 and 7.4, respectively, GSI and GSII had similar molecular weight of 367,000, and both enzymes consisted of eight identical subunits with molecular weight of 45,800. In the presence of Mg2+, the activities of GSI and GSII were strongly inhibited by Mn1*. Moderate inhibition was observed by Fe2+, Ca2+ and Zn2+. However, Co2+ stimulated the activities of GSI and GSII markedly.