ASPARAGINE METABOLISM AND NITROGEN DISTRIBUTION DURING PROTEIN-DEGRADATION IN SUGAR-STARVED MAIZE ROOT-TIPS
ASPARAGINE METABOLISM AND NITROGEN DISTRIBUTION DURING PROTEIN-DEGRADATION IN SUGAR-STARVED MAIZE ROOT-TIPS
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DOI：
10.1007/bf00192806
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发表时间：
1992-10-01
期刊：
影响因子：
4.3
通讯作者：
RAYMOND, P
中科院分区：
文献类型：
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作者：
BROUQUISSE, R;JAMES, F;RAYMOND, P
离体玉米（Zea mays L.）以根尖为材料，研究了长时间葡萄糖饥饿对氮代谢的影响。根尖切除后，糖含量迅速耗尽，蛋白质含量下降到40和8%，其初始值后96和192小时，分别。在饥饿过程中，游离氨基酸含量发生变化。属于同一“合成家族”的氨基酸表现出类似的变化模式，表明它们的含量在饥饿期间主要控制在它们共同的生物合成步骤的水平上。天冬酰胺，这是一个很好的标志物的蛋白质和氨基酸的降解压力条件下，积累相当大，直到45小时的饥饿，并占50%的蛋白质降解释放的氮在那个时候。饥饿45小时后，氮停止储存在天冬酰胺中，并从细胞中排出，首先作为氨排出，直到90-100小时，然后，当饥饿变得不可逆时，作为氨基酸和胺化化合物排出。对饥饿过程中天冬酰胺代谢和氮同化途径的研究表明：（1）天冬酰胺合成是通过天冬酰胺合成酶（EC 6.3）进行的。1. 1)而不是天冬酰胺氨基转移酶（EC 2.6.1.14）或β-氰基丙氨酸途径，并且天冬酰胺降解通过天冬酰胺酶（EC 3.5.1.1）发生;（ii）在饥饿恢复期，与氮还原、同化和氨基酸合成有关的酶活性持续下降，而谷氨酸脱氢酶（EC 1.4.1.2-4）活性上升。综合考虑，代谢物分析和酶活性测量表明，饥饿可分为三个阶段：（一）适应阶段（0 ~ 30 ~ 35 h），根尖适应短暂缺糖，部分贮存蛋白质降解释放的氮，（二）生存期（30-35至90-100 h），在此期间根尖排出蛋白质降解释放的氮，并且饥饿可以通过添加糖来逆转;（iii）细胞解体阶段（超过100 h），其中所有代谢物和酶活性降低，根尖死亡。
Excised maize (Zea mays L.) root tips were used to monitor the effects of prolonged glucose starvation on nitrogen metabolism. Following root-tip excision, sugar content was rapidly exhausted, and protein content declined to 40 and 8% of its initial value after 96 and 192 h, respectively. During starvation the contents of free amino acids changed. Amino acids that belonged to the same "synthetic family" showed a similar pattern of changes, indicating that their content, during starvation, is controlled mainly at the level of their common biosynthetic steps. Asparagine, which is a good marker of protein and amino-acid degradation under stress conditions, accumulated considerably until 45 h of starvation and accounted for 50% of the nitrogen released by protein degradation at that time. After 45 h of starvation, nitrogen ceased to be stored in asparagine and was excreted from the cell, first as ammonia until 90-100 h and then, when starvation had become irreversible, as amino acids and aminated compounds. The study of asparagine metabolism and nitrogen-assimilation pathways throughout starvation showed that: (i) asparagine synthesis occurred via asparagine synthetase (EC 6.3. 1. 1) rather than asparagine aminotransferase (EC 2.6.1.14) or the beta-cyanoalanine pathway, and asparagine degradation occurred via asparaginase (EC 3.5.1.1); and (ii) the enzymic activities related to nitrogen reduction and assimilation and amino-acid synthesis decreased continuously, whereas glutamate dehydrogenase (EC 1.4.1.2-4) activities increased during the reversible period of starvation. Considered together, metabolite analysis and enzymic-activity measurements showed that starvation may be divided into three phases: (i) the acclimation phase (0 to 30-35 h) in which the root tips adapt to transient sugar deprivation and partly store the nitrogen released by protein degradation, (ii) the survival phase (30-35 to 90-100 h) in which the root tips expel the nitrogen released by protein degradation and starvation may be reversed by sugar addition and (iii) the cell-disorganization phase (beyond 100 h) in which all metabolites and enzymic activities decrease and the root tips die.