Some regulative properties of glutamine synthetase in cucumber leaves

Abstract

Abstract Glutamine synthetase (L-glutamate: ammonia ligase (ADP) EC 6. 3. t 2) was prepared from cucumber leaves grown on ammonium medium and some regulative properties were investigated. The apparent Michaelis constant (Km) values of the various substrates and cofactors were determined. Lineweaver-Burk plots gave a Km of 4.5mM with L-glutamate, 0.74mM with ATP and 3.1 mM with NH2OH. Metals, including Cu2+, Hg2+, Zn2+, Ni2+, Fe2+, and Fe3+, strongly inhibited the enzyme activity, Cd2+ did not inhibit the enzyme activity markedly, in contrast with its effect on the rice enzyme. Ca2+ was quite inhibitive to enzyme reaction and more than 50% of the activity was lost at 3 mM. The amino acids tested generally had no effect on the enzyme activity except alanine, which showed little but clear inhibition. Isocitrate and α-ketoglutarate were slightly promotive to enzyme activity while pyruvate and glyoxylate (24 mM) significantly inhibited the enzyme activity. Glucose-l- or -6-phosphate and fructose-6-phosphate we...