Urease Is Not Essential for Ureide Degradation in Soybean
Urease Is Not Essential for Ureide Degradation in Soybean
复制标题
脲酶并不是大豆脲化物降解所必需的
作者：
Norm;E.;Stebbins;J. Polacco
假设大豆(Glyine max L.)[Merrill])在生理意义上将尿素分解为尿素的方法被测试并拒绝。尿素酶阴性(EU3-E1/EU3-E1)植株被固定的氮气或2 mM的NH4NO3支持，因此木质部携带的氮分别含有主要的酰脲(尿囊素和尿囊酸)或酰胺氨基酸。种子氮素产量在两个氮素水平上相同，尽管35d龄的固定植株积累的叶尿素大约是后者的6倍。在愈伤组织中，缺乏活性的尿素酶抑制了精氨酸或尿囊素作为唯一氮源的生长，但精氨酸(73%)比尿囊素(39%)的降幅更大。此外，尿素酶阴性的细胞在精氨酸上积累的尿素是尿素酶阳性细胞的17倍；尿囊素的比率是1.8。尿素酶阴性愈伤组织以3%的出苗率积累尿素。为了测试尿素酶阴性幼苗中积累的尿素是否来自脲类化合物，种子首先被允许在1 mM别嘌醇中吸收，别嘌醇是一种脲形成的抑制剂。苗木缩脲降低了90%，但尿素水平没有变化。因此，脲是尿素的不良前体，这在种子吸收的[14C-尿素]尿囊酸转化为[14C]尿素的苗木中得到证实，而[14C-胍]精氨酸的40%-70%被回收为[14C]尿素。
The hypothesis that soybean (Glycine max L. [Merrill]) catabolizes ureides to urea to a physiologically significant extent was tested and rejected. Urease-negative (eu3-e1/eu3-e1) plants were supported by fixed N2 or by 2 mM NH4NO3, so that xylem-borne nitrogen contained predominantly ureides (allantoin and allantoic acid) or amide amino acids, respectively. Seed nitrogen yield was equal on either nitrogen regime, although 35-d-old fixing plants accumulated about 6 times more leaf urea. In callus, lack of an active urease reduced growth on either arginine or allantoin as the sole nitrogen source, but the reduction was greater on arginine (73%) than on allantoin (39%). Furthermore, urease-negative cells accumulated 17 times more urea than urease-positive cells on arginine; for allantoin the ratio was 1.8. Urease-negative callus accumulated urea at 3% the rate of seedlings. To test whether urea accumulating in urease-negative seedlings was derived from ureides, seeds were first allowed to imbibe in 1 mM allopurinol, an inhibitor of ureide formation. Seedling ureides were decreased by 90%, but urea levels were unchanged. Thus, ureides are poor precursors of urea, which was confirmed in seedlings that converted no more than 5% of seed-absorbed [14C-ureido]allantoate to [14C]urea, whereas 40 to 70% of [14C-guanido]arginine was recovered as [14C]urea.