title: Purified gamma-glutamyl transpeptidases from tomato exhibit high affinity for glutathione and glutathione S-conjugates.
abstract: gamma-Glutamyl transpeptidases (gammaGTases) are the only enzymes known to hydrolyze the unique N-terminal amide bonds of reduced glutathione (gamma-L-glutamyl-cysteinyl-glycine), oxidized glutathione, and glutathione S-conjugates. Two gammaGTases (I and II) with K(m) values for glutathione of 110 and 90 microM were purified 2,977-fold and 2,152-fold, respectively, from ripe tomato (Lycopersicon esculentum) pericarp. Both enzymes also hydrolyze dipeptides and other tripeptides with N-terminal, gamma-linked Glu and the artificial substrates gamma-L-glutamyl-p-nitroanilide and gamma-L-glutamyl(7-amido-4-methylcoumarin). They transfer the glutamyl moiety to water or acceptor amino acids, including L-Met, L-Phe, L-Trp, L-Ala, or the ethylene precursor 1-aminocyclopropane-1-carboxylic acid. gammaGTase I and II were released from a wall and membrane fraction of a tomato fruit extract with 1.0 M NaCl, suggesting that they are peripheral membrane proteins. They were further purified by acetone precipitation, Dye Matrex Green A affinity chromatography, and hydrophobic interaction chromatography. The two gammaGTases were resolved by concanavalin A (Con A) affinity chromatography, indicating that they are differentially glycosylated. The native and SDS-denatured forms of both enzymes showed molecular masses of 43 kD.
authors.affiliation: Climate Stress Laboratory, Beltsville Agricultural Research Center, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland 20705, USA. mmartin@aesop.rutgers.edu
authors.forename: M N
authors.initials: MN
authors.lastname: Martin
authors.forename: J P
authors.initials: JP
authors.lastname: Slovin
mesh_terms: D005978:Glutathione / Q000031:analogs & derivatives / Q000378:metabolism*; D018551:Solanum lycopersicum / Q000201:enzymology*; D013379:Substrate Specificity; D005723:gamma-Glutamyltransferase / Q000302:isolation & purification / Q000378:metabolism*
chemical_list: D005723:gamma-Glutamyltransferase; D005978:Glutathione