Takeuchi Y, Tsumuraya Y
Department of Biochemistry and Molecular Biology, Faculty of Science, Saitama University, Japan.
Biosci Biotechnol Biochem. 2001 Jul;65(7):1519-27. doi: 10.1271/bbb.65.1519.
A membrane preparation of 7-d-old seedlings from azuki bean (Vigna angularis) contained galacturonosyltransferase (GalAT) capable of transferring galacturonic acid (GalA) from UDP-GalA into polygalacturonic acid (PGA) as an exogenous acceptor. The enzyme was maximally active at pH 6.8-7.8 and 25-35 degrees C in the presence of 5 mM Mn2+ and 0.5% (w/v) Triton X-100. Acid-soluble low-Mr (average Mr 10,000) PGA was a more efficient acceptor substrate than acid-insoluble polymer (Mr 70,000). The apparent Michaelis constants for UDP-GalA and low-Mr PGA were 0.14 mM and 0.02 mg/ml, respectively. Various pectins with different degrees of methyl-esterification (DE) were poor acceptors, and the enzyme activity tended to decrease with decreasing DE of the pectins. The transfer products from incubation of the enzyme with UDP-14C-GalA and the low-Mr PGA yielded 14C-GalA2 as the major product upon digestion with an endopolygalacturonase (EPGase), confirming the incorporation of GalA into PGA through contiguous alpha-1,4-linkages.
来自赤豆（Vigna angularis）7日龄幼苗的膜制剂含有半乳糖醛酸基转移酶（GalAT），该酶能够将半乳糖醛酸（GalA）从UDP - GalA转移到作为外源受体的聚半乳糖醛酸（PGA）中。在5 mM Mn2+和0.5%（w/v） Triton X - 100存在的情况下，该酶在pH 6.8 - 7.8和25 - 35摄氏度时活性最高。酸溶性低Mr（平均Mr 10,000）的PGA比酸不溶性聚合物（Mr 70,000）是更有效的受体底物。UDP - GalA和低Mr PGA的表观米氏常数分别为0.14 mM和0.02 mg/ml。各种不同甲酯化程度（DE）的果胶是较差的受体，并且酶活性倾向于随着果胶DE的降低而降低。用内切聚半乳糖醛酸酶（EPGase）消化该酶与UDP - 14C - GalA和低Mr PGA孵育的转移产物，产生14C - GalA2作为主要产物，证实了GalA通过连续的α - 1,4 - 连接掺入PGA中。
