首页 | 本学科首页   官方微博 | 高级检索  
   检索      


Characterization of glutamine synthetase in the apple
Authors:Seong-Mo Kang  John S Titus
Institution:Department of Horticulture, University of Illinois, Urbana, Illinois 61801, U.S.A.
Abstract:Glutamine synthetase (l -glutamate: ammonia ligase, ADP-forming, EC 6.3.1.2) in bark tissue of the apple (Malus domestica Borkh. cv. Golden Delicious) was partially purified and characterized. The Mn2+- and Mg2+-dependent activities were maximal at pH 7.2 and 7.5, respectively. The enzyme was almost completely inactivated within two weeks at 0°C. Both Mg2+ and β-mercaptoethanol were effective in stabilizing the enzyme during storage. The enzyme was protected from thermal inactivation at 60°C by the addition of Mg2+ and ATP. One-tenth mM phenylmercuric acetate inhibited the Mg2+-dependent activity by 50%. Equimolar dithiothreitol protected the enzyme from this inactivation. The Km values of the enzyme were 0.27, 7.35, and 0.69 mM for ATP, glutamate, and NH2OH, respectively. The constant for NH+4 was an order of magnitude higher in the presence of Mn2+ than Mg2+. When the amino acids were externally added to the reaction mixtures, the measurement of Pi exhibited a higher degree of enzyme inhibition than the measurement of γ-glutamyl monohydroxamate (GHA). Ten mM histidine inhibited the Mg2+- and Mn2+-dependent activities by 26 and 45% respectively. Twenty mM aspartate (d,l -form) inhibited the enzyme 30% in the presence of either Mg2+ or Mn2+. Aspartate (Mg2+-dependent) and histidine (Mn2+-dependent) inhibited the enzyme competitively with respect to glutamate, the estimated inhibition constants being 17.6 and 1.6 mM, respectively. At 10 mM, amino acids such as tryptophan, arginine, alanine and citrulline inhibited enzyme activity from 1 to 18%. Glutamine stimulated the Mg2+-dependent activity 25% at 25 mM when GHA was measured. Glutamine above 32 mM inhibited the enzyme.
Keywords:Malus domestica            amino acids  nitrogen  cation  enzyme inhibition
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号