U.S. Department of Agriculture/Agricultural Research Service, University of Minnesota, St. Paul, Minnesota 55108.
Plant Physiol. 1989 Aug;90(4):1622-9. doi: 10.1104/pp.90.4.1622.
Aspartate aminotransferase (l-aspartate:2-oxoglutarate aminotransferase, EC 2.6.1.1 [AAT]), a key enzyme in the assimilation of C and N compounds, was purified from the cytosol of alfalfa (Medicago sativa L.) root nodules. Isoforms that increased during nodule development, AAT-2a, AAT-2b, and AAT-2c, were purified greater than 447-fold to apparent homogeneity, and high titer polyclonal antibodies were produced. The native molecular weight of the AAT-2 isoforms was approximately 80 kilodatons with a subunit molecular weight of 40 kilodatons, indicating that the holoenzymes are dimers. The AAT-2 isoforms comprised approximately 0.4% of the total soluble nodule protein. The AAT specific activity was measured in leaf, stem, root, and nodule organs, and zymograms of each were compared. Enzyme activity was 4- to 37-fold greater in effective (nitrogen fixing) nodules than in leaves, stems, and roots. Effective nodule AAT-specific activity was 3- to 8-fold greater than that of plant-controlled ineffective nodules. No differences in K(m) were observed between AAT-1 and AAT-2. Antibodies raised against AAT-2 were more selective against AAT-2 than AAT-1. Evidence obtained from zymograms suggests that the expression of alfalfa nodule AAT is controlled at two different gene loci, AAT-1 and AAT-2, resulting in different dimeric isoforms.
天冬氨酸氨基转移酶（l-天冬氨酸：2-氧代戊二酸氨基转移酶，EC 2.6.1.1[AAT]）是同化 C 和 N 化合物的关键酶，从紫花苜蓿（Medicago sativa L.）根瘤的胞质中纯化出来。在根瘤发育过程中增加的同工酶 AAT-2a、AAT-2b 和 AAT-2c 被纯化了 447 倍以上，达到明显的均一性，并产生了高滴度的多克隆抗体。AAT-2 同工酶的天然分子量约为 80 千道尔顿，亚基分子量为 40 千道尔顿，表明全酶是二聚体。AAT-2 同工酶约占总可溶性根瘤蛋白的 0.4%。在叶片、茎、根和根瘤器官中测定了 AAT 的比活性，并比较了每个器官的同工酶图谱。酶活性在有效（固氮）根瘤中比叶片、茎和根中高 4 到 37 倍。有效根瘤 AAT 比活性比植物控制的无效根瘤高 3 到 8 倍。在 AAT-1 和 AAT-2 之间没有观察到 K(m)的差异。针对 AAT-2 产生的抗体对 AAT-2 的选择性比对 AAT-1 更高。同工酶图谱获得的证据表明，紫花苜蓿根瘤 AAT 的表达受两个不同的基因座 AAT-1 和 AAT-2 控制，导致不同的二聚体同工酶。
