Degradation of glutathione in plant cells: Evidence against the participation of a γ-glutamyltranspeptidase

Abstract

When γ-glutamyltranspeptidase activity in tobacco cells was measu red using the artificial substrate γ-glutamyl-p-nitroanilide, liberation of p-nitroaniline was not reduced, but stimulated by addition of glutathione.Therefore, glutathione was not acting as a donator, but as an acceptor of γ-glutamylmoieties in the assay mixture, suggesting th at γ-glutamyltran spetidase is not participating in degradation of glutathione. Feeding experiments with [35S -cysJglutathione supported this conclusion.When tobacco cells were supplied with this peptide as sole sulfur source, glutathioneand γ-glutamylcysteine were the only labelled compounds found inside the cells.The low rate of uptake of glutathione apparently prevented the accumulation of measurable amounts of radioactivity in the cysteine pool.A γ-glutamylcyclotransferase, responsible for the conversion of γ-glutamylcysteine to 5-oxo-proline and cysteine was found in ammonium sulfatep recipitates of tobacco cell hom ogenates.The enzyme showed high activities with γ-glutamylmethionine and γ-glutamylcysteine, but not with other γ-glutamyldipeptides or glutathione.From these and previously published experiments [(Rennenbergetal., Z.Naturforsch.35c, 708 -711 (1980)], it is concluded that glutathione is degraded in tobacco cellsvia the following path way: γ-glu-cysgly → γ-glu-cys → · 5-oxo-proline → glu. © 1985, Walter de Gruyter. All rights reserved.