Biology Department, McMaster University, L8S 4Kl, Hamilton, Ont.
Planta. 1979 Jan;146(2):237-41. doi: 10.1007/BF00388238.
An asparagine synthetase which is active with either glutamine or NH 4 (+) has been found in maize (Zea mays L.) roots. Unlike the enzyme obtained from legume cotyledons, the maize-root enzyme is only slightly more efficient with glutamine (Km, 1.0 mM) than with NH 4 (+) (Km, 2.0-3.0 mM). The activity of this enzyme is higher in the mature root than in the root-tip region, i.e. root cells develop a capacity to make asparagine from glutamine or NH 4 (+) as they mature. β-Cyanoalanine synthetase is also present in maize roots. The apparent Km for cysteine is 2.6 mM and for cyanide is 0.57 mM. The enzyme is more active in the root tip than in mature root tissue. Thus, if asparagine were made in the root tip, the cyanide pathway could represent the mechanism of synthesis. It is our contention, however, that this potential is not realized under normal conditions because (14)C-experiments performed previously have indicated a limited availability of both CN and cysteine in the maize root.
在玉米（Zea mays L.）根中发现了一种能够与谷氨酰胺或 NH 4 (+) 反应的天冬酰胺合成酶。与从豆科子叶中获得的酶不同，玉米根酶对谷氨酰胺（Km，1.0 mM）的效率仅略高于 NH 4 (+)（Km，2.0-3.0 mM）。该酶在成熟根中的活性高于根尖区域，即根细胞在成熟过程中从谷氨酰胺或 NH 4 (+) 中产生天冬酰胺的能力增加。β-氰基丙氨酸合成酶也存在于玉米根中。半胱氨酸的表观 Km 为 2.6 mM，氰化物为 0.57 mM。该酶在根尖的活性高于成熟根组织。因此，如果在根尖合成天冬酰胺，那么氰化物途径可能代表合成的机制。然而，我们认为，在正常条件下，这种潜力并没有实现，因为之前进行的 14C 实验表明，玉米根中 CN 和半胱氨酸的可用性有限。
