Purification and Properties of Glutamine Synthetase from Germinating Castor Bean Endosperm

The activity of glutamine synthetase in germinating castor bean endosperm resides in the cytosol. The activity was found to increase rapidly the 3rd to 4th day of germanating in the dark and to be a peak at 5th day. The enzyme was purified from 5-day-old endosperm approximately 100-fold to apparent homogeneity. Electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate yeilded a single band with a molecular weight of 55,000, suggesting that subunits of GS have a identical molecular weight. The Km is 4.9mM for L-glutamate, 0.65mM for NH_2OH, 0.92mM for NH_4Cl (biosynthetic assay), 34.1mM for L-glutamine and 6.28mM for NH_2OH (transferase assay). ATP acts as a sigmoidal substrate with a "S_<0.5>" of 1.4mM. Inhibition of the enzyme is observed with AMP, ADP, CTP, GTP, ITP, L-glycine, L-histidine, citrate, carbamyl phsphate, and glucoseamine-6-phosphate. The energe charge significantly affected the activity of GS