Received July 3, 2008
Accepted August 29, 2008
A 
-Glutamyl Transpeptidase-Independent Pathway of Glutathione Catabolism to Glutamate via 5-Oxoproline in Arabidopsis
Naoko Ohkama-Ohtsu , Akira Oikawa , Ping Zhao , Chengbin Xiang , Kazuki Saito , and David J. Oliver *
Department of Genetics, Development and Cell Biology, Iowa State University, Ames, IA 50011 USA; RIKEN Plant Science Center, Yokohama 230–0045, Japan; School of Life Sciences, University of Science and Technology of China, Hefei, Auhui 230027 China
* Corresponding author; email: doliver{at}iastate.edu.
The degradation pathway of glutathione (GSH) in plants is not well understood. In mammals, GSH is predominately metabolized through the 
-glutamyl cycle, where GSH is degraded by the sequential reaction of -glutamyl transpeptidase (GGT), -glutamyl cyclotransferase (GGC) and 5-oxoprolinase (5OPase) to yield Glu and dipeptides that are subject to peptidase action. In this study we examined if GSH is degraded through the same pathway in Arabidopsis thaliana as occurs in mammals. In Arabidopsis the oxoprolinase knockout mutants (oxp1-1 and oxp1-2) accumulate more 5-oxoproline (5OP) and less Glu than wild-type plants, suggesting substantial metabolite flux though 5OP and that 5OP is a major contributor to Glu steady state levels. In the ggt1-1/ggt4-1/oxp1-1 triple mutant with no GGT activity in any organs except young siliques, the 5OP concentration in leaves was not different from that in oxp1-1 suggesting that GGTs are not major contributors to 5OP production in Arabidopsis. 5OP formation strongly tracked the level of GSH in Arabidopsis plants suggesting that GSH is the precursor of 5OP in a GGT-independent reaction. Kinetics analysis suggests that -glutamyl cyclotransferase is the major source of GSH degradation and 5OP formation in Arabidopsis. This discovery has led us to propose a new pathway for GSH turnover in plants where GSH is converted to 5OP and then glutamate by the combined action of -glutamyl cyclotransferase and 5-oxoprolinase in the cytoplasm.
