γ-Glutamyltransferases in higher plants and catabolism of glutathiones

γ-谷氨酰转移酶（GGT）催化γ-谷氨酰基肽（如谷胱甘肽）γ-谷氨酰基部分的水解和转肽化。在这项研究中，我们发现了以下几点。(1)萝卜（Raphanus sativus）子叶亚细胞分离和液泡分离表明可溶性GGT为液泡酶。acivvicin是一种GGT抑制剂，它介导了由内源性谷胱甘肽和外源性单氯比烷产生的谷胱甘肽s缀合物的体内分解代谢抑制。因此，可溶性GGT可能参与谷胱甘肽和谷胱甘肽s缀合物的分解代谢。(2)用萝卜全长cdna （RsGGT1、RsGGT2和RsGGT3）编码推定的异二聚体GGT异构体转化烟草植株。表达分析表明，RsGGTs具有不同的器官表达模式。在转基因烟草植株中，RsGGT1和rsggt2cdna过表达，而RsGGT3cDNA不过表达，导致nacl可提取结合GGT活性增加。这些结果表明，RsGGT1和rsggt2cdna编码异二聚体结合的GGT亚型。(3)拟南芥突变体在假定的GGT基因AtGGT1、AtGGT2和AtGGT3中含有T-DNA或转座子插入。基因表达分析表明，鉴定的atggt1-1、atggt2-1、atggt2-2和atggt3-1植株为GGT-null突变体。GGT活性分析表明，atggt1突变等位基因缺乏细胞壁结合型GGT，而两个atggt2突变等位基因和atggt3突变等位基因缺乏可溶性GGT。野生型和atggt1植物的谷胱甘肽含量和谷胱甘肽分解率表明atggt1参与了谷胱甘肽的分解代谢。AtGGT1, 2,3似乎对应于rsggt - 1,2,3。根据它们的顺序。然而，这些基因之间仍然存在差异。

γ-Glutamyltransferase (GGT) catalyzes the hydrolysis and transpeptidation of the γ-glutamyl moiety of γ-glutamyl peptides such as glutathione. We found the followings in this study.(1) Subcellular fractionation of radish (Raphanus sativus) cotyledon and isolation of vacuole revealed that soluble GGT is a vacuolar enzyme. Acivicin, a GGT inhibitor, mediated the in vivo catabolism inhibition of the glutathione S-conjugate generated from endogenous glutathione and exogenously supplied monochlorobimane. Thus, soluble GGT is possibly involved in the catabolism of glutathione and glutathione S-conjugates.(2) Tobacco plants transformed with□□ radish full-length cDNAs (RsGGT1, RsGGT2 and RsGGT3) encoding putative heterodimeric GGT isoforms. Expression analysis indicated that RsGGTs showed different organ expression patterns. The overexpression of RsGGT1 and RsGGT2cDNAs, but not that of RsGGT3cDNA, resulted in an increase of NaCl-extractable bound GGT activity in transgenic tobacco plants. These results suggest that RsGGT1 and RsGGT2cDNAs encode heterodimeric bound GGT isoforms.(3) Arabidopsis mutants harboring T-DNA or transposon insertions in putative GGT genes AtGGT1, AtGGT2 and AtGGT3 were characterized. Gene expression analysis indicated that the identified atggt1-1, atggt2-1, atggt2-2 and atggt3-1 plants were GGT-null mutants. GGT activity analysis indicated that the atggt1 mutant allele was deficient in cell wall bound GGT, whereas the two atggt2 mutant alleles and the atggt3 mutant allele were deficient in soluble GGT. GSH contents and GSH catabolic rates in wild type and atggt1 plants suggest that AtGGT1 is involved in GSH catabolism. AtGGT1, 2, 3 seem to be corresponding to RsGGT-1, 2, 3. based on their sequences. However there still be discrepancy between these genes.