title: Inhibition of rat liver glutathione S-transferase isoenzymes by peptides stabilized against degradation by gamma-glutamyl transpeptidase.
abstract: Inhibitors for glutathione S-transferase (GST) iso-enzymes from rat liver with high affinity for the glutathione-binding site (G-site) have been developed. In previous studies, a model was described for the G-site of GST (Adang, A. E. P., Brussee, J., van der Gen, A., and Mulder, G. J. (1990) Biochem. J. 269, 47-54) in terms of essential and nonessential interactions between groups in glutathione (GSH) and the G-site. Based on this model, compounds were designed that have high affinity for the G-site but cannot be conjugated. In the dipeptide gamma-L-glutamyl-D-aminoadipic acid (gamma-L-Glu-D-Aad), the L-cysteinylglycine moiety is replaced by D-aminoadipic acid. This dipeptide is an efficient competitive inhibitor (toward GSH) of mu class GST isoenzymes with Ki values of 34 microM for GST isoenzyme 3-3 and 8 microM for GST isoenzyme 4-4. Other GSH-dependent enzymes, such as gamma-glutamyl transpeptidase (gamma-GT), glutathione reductase, and glutathione peroxidase, were not inhibited by 1 mM of gamma-L-Glu-D-Aad. Inhibition is also highly stereospecific since gamma-L-Glu-L-Aad is only a poor inhibitor (Ki = 430 microM for GST 3-3). Gamma-L-Glutamyl-D-norleucine also had a much higher Ki value for GST 3-3. Thus, the presence of a delta-carboxylate group in D-Aad appears to be essential for a high affinity inhibitor. An additional hydrophobic group did not result in increased inhibitory potency. In a different approach, the gamma-L-glutamyl moiety in GSH was replaced by delta-L-aminoadipic acid; delta-L-Aad-L-Cys-Gly is an efficient cosubstrate analogue for GSTs with Km values comparable to GSH and Vmax values ranging from 0.24 to 57 mumol/min/mg for the different GSTs. The structures of the efficient inhibitor and the cosubstrate analogue were combined in delta-L-Aad-D-Aad, which had a Ki value of 68 microM with GST 3-3. In order to investigate their possible use in vivo studies, the degradation of gamma-L-Glu-D-Aad and delta-L-Aad-L-Cys-Gly by gamma-GT was investigated. The peptides showed no measurable hydrolysis rates under conditions where GSH was rapidly hydrolyzed. Thus, an efficient, mu class-specific GST inhibitor and a gamma-glutamyl-modified cosubstrate analogue of GSH were developed. Their gamma-GT stability offers the possibility to use these peptides in in vivo experiments.
authors.affiliation: Division of Toxicology, University of Leiden, The Netherlands.
authors.forename: A E
authors.initials: AE
authors.lastname: Adang
authors.forename: J
authors.initials: J
authors.lastname: Brussee
authors.forename: A
authors.initials: A
authors.lastname: van der Gen
authors.forename: G J
authors.initials: GJ
authors.lastname: Mulder
mesh_terms: D000818:Animals; D005982:Glutathione Transferase / Q000037:antagonists & inhibitors* / Q000302:isolation & purification / Q000378:metabolism; D007527:Isoenzymes / Q000037:antagonists & inhibitors* / Q000302:isolation & purification / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D010455:Peptides / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D051381:Rats; D005723:gamma-Glutamyltransferase / Q000378:metabolism*
chemical_list: D007527:Isoenzymes; D010455:Peptides; D005723:gamma-Glutamyltransferase; D005982:Glutathione Transferase