Probing the active site of alpha-class rat liver glutathione S-transferases using affinity labeling by monobromobimane

The evaluation of novel natural products as inhibitors of human glutathione transferase P1-1

Glutathione transferase P1-1 is over expressed in some cancer cells and contributes to detoxification of anticancer drugs, leading to drug-resistant tumors. The inhibition of human recombinant GSTP1-1 by natural plant products was investigated using 10 compounds isolated from plants indigenous to Southern and Central Africa. Monochlorobimane and 1-chloro-2,4-dinitrobenzene were used to determine GST activity. Each test compound was screened at 33 and 100 µM. Isofuranonapthoquinone (1) (from Bulbine frutescens) showed 68% inhibition at 33 µM, and sesquiterpene lactone (2) (from Dicoma anomala) showed 75% inhibition at 33 μM. The IC(50) value of 1 was 6.8 μM. The mode of inhibition was mixed, partial (G site) and noncompetitive (H site) with K(i) values of 8.8 and 0.21 µM, respectively. Sesquiterpene 2 did not inhibit the CDNB reaction. Therefore, isofuranonapthoquinone 1 needs further investigations in vivo because of its potent inhibition of GSTP1-1 in vitro.

Contribution of the mu loop to the structure and function of rat glutathione transferase M1-1

The "mu loop," an 11-residue loop spanning amino acid residues 33-43, is a characteristic structural feature of the mu class of glutathione transferases. To assess the contribution of the mu loop to the structure and function of rat GST M1-1, amino acid residues 35-44 (35GDAPDYDRSQ44) were excised by deletion mutagenesis, resulting in the "Deletion Enzyme." Kinetic studies reveal that the Km values of the Deletion Enzyme are markedly increased compared with those of the wild-type enzyme: 32-fold for 1-chloro-2,4-dinitrobenzene, 99-fold for glutathione, and 880-fold for monobromobimane, while the Vmax value for each substrate is increased only modestly. Results from experiments probing the structure of the Deletion Enzyme, in comparison with that of the wild-type enzyme, suggest that the secondary and quaternary structures have not been appreciably perturbed. Thermostability studies indicate that the Deletion Enzyme is as stable as the wild-type enzyme at 4 degrees C and 10 degrees C, but it rapidly loses activity at 25 degrees C, unlike the wild-type enzyme. In the temperature range of 4 degrees C through 25 degrees C, the loss of activity of the Deletion Enzyme is not the result of a change in its structure, as determined by circular dichroism spectroscopy and sedimentation equilibrium centrifugation. Collectively, these results indicate that the mu loop is not essential for GST M1-1 to maintain its structure nor is it required for the enzyme to retain some catalytic activity. However, it is an important determinant of the enzyme's affinity for its substrates.

Delineation of xenobiotic substrate sites in rat glutathione S-transferase M1-1

Glutathione S-transferases catalyze the conjugation of glutathione with endogenous and exogenous xenobiotics. Hu and Colman (1995) proposed that there are two distinct substrate sites in rat GST M1-1, a 1-chloro-2,4-dintrobenzene (CDNB) substrate site located in the vicinity of tyrosine-115, and a monobromobimane (mBBr) substrate site. To determine whether the mBBr substrate site is distinguishable from the CDNB substrate site, we tested S-(hydroxyethyl)bimane, a nonreactive derivative of mBBr, for its ability to compete kinetically with the substrates. We find that S-(hydroxyethyl)bimane is a competitive inhibitor (K(I) = 0.36 microM) when mBBr is used as substrate, but not when CDNB is used as substrate, demonstrating that these two sites are distinct. Using site-directed mutagenesis, we have localized the mBBr substrate site to an area midway through alpha-helix 4 (residues 90-114) and have identified residues that are important in the enzymatic reaction. Substitution of alanine at positions along alpha-helix 4 reveals that mutations at positions 103, 104, and 109 exhibit a greater perturbation of the enzymatic reaction with mBBr than with CDNB as substrate. Various other substitutions at positions 103 and 104 reveal that a hydrophobic residue is necessary at each of these positions to maintain optimal affinity of the enzyme for mBBr and preserve the secondary structure of the enzyme. Substitutions at position 109 indicate that this residue is important in the enzyme's affinity for mBBr but has a minimal effect on Vmax. These results demonstrate that the promiscuity of rat GST M1-1 is in part due to at least two distinct substrate sites.

Monobromobimane occupies a distinct xenobiotic substrate site in glutathione S-transferase pi

Monobromobimane (mBBr), functions as a substrate of porcine glutathione S-transferase pi (GST pi): The enzyme catalyzes the reaction of mBBr with glutathione. S-(Hydroxyethyl)bimane, a nonreactive analog of monobromobimane, acts as a competitive inhibitor with respect to mBBr as substrate but does not affect the reaction of GST pi with another substrate, 1-chloro-2,4-dinitrobenzene (CDNB). In the absence of glutathione, monobromobimane inactivates GST pi at pH 7.0 and 25 degrees C as assayed using mBBr as substrate, with a lesser effect on the enzyme's use of CDNB as substrate. These results indicate that the sites occupied by CDNB and mBBr are not identical. Inactivation is proportional to the incorporation of 2 moles of bimane/mole of subunit. Modification of GST pi with mBBr does not interfere with its binding of 8-anilino-1-naphthalene sulfonate, indicating that this hydrophobic site is not the target of monobromobimane. S-Methylglutathione and S-(hydroxyethyl)bimane each yield partial protection against inactivation and decrease reagent incorporation, while glutathionyl-bimane protects completely against inactivation. Peptide analysis after trypsin digestion indicates that mBBr modifies Cys45 and Cys99 equally. Modification of Cys45 is reduced in the presence of S-methylglutathione, indicating that this residue is at or near the glutathione binding region. In contrast, modification of Cys99 is reduced in the presence of S-(hydroxyethyl)bimane, suggesting that this residue is at or near the mBBr xenobiotic substrate binding site. Modification of Cys99 can best be understood by reaction with monobromobimane while it is bound to its xenobiotic substrate site in an alternate orientation. These results support the concept that glutathione S-transferase accomplishes its ability to react with a diversity of substrates in part by harboring distinct xenobiotic substrate sites.

Heterodimers of glutathione S-transferase can form between isoenzyme classes pi and mu

Glutathione S-transferases constitute a family of enzymes that detoxify xenobiotics by conjugating glutathione with a range of electrophilic substrates. The cytosolic glutathione S-transferase dimeric isoenzymes are currently divided into at least eight classes on the basis of their physical and chemical properties. Previously, heterodimers have only been detected within a given class of isoenzymes; however, here we describe for the first time the generation of a heterodimer between a pi class and mu class glutathione S-transferase. The heterodimer forms under mild conditions (dialysis against phosphate buffer, pH 6.5) and is best detected when one of the isoenzyme subunits is in excess. The activity of the pi-mu heterodimer toward several substrates indicates that interaction between these two dissimilar subunits influences the catalytic activity of this dimer. The production of this new heterodimer provides a new approach in glutathione S-transferase research to study the influence of one subunit on the catalytic activity of its partner subunit and to identify those amino acid residues which contribute to subunit interactions.

Discriminative disulfide-bonding affinity labeling of opioid receptor subtypes

The affinity-labeling technique is an extremely important method in receptor biochemistry. The 3-nitro-2-pyridinesulfenyl (Npys) group, attached to a mercapto group, can react only with a free thiol group (the beta-mercapto group of cysteine residue) of the target receptor molecules, forming a disulfide bond. This disulfide bonding is mediated through the thiol-disulfide exchange reaction. Unlike other labeling methods, the approach utilizing such chemically activated thiol-containing ligands is able to reproduce an unlabeled protein by treatment with dithiothreitol, a reducing reagent. This provides several unique aspects for the studies elucidating the structure-function relationships between the peptide and the receptor. Based on the SNpys affinity technique, we have achieved the discriminative disulfide-bonding affinity labeling of the three different subtypes of opioid receptors: mu, delta and kappa. This article reviews our novel affinity techniques in the in vitro receptor biochemistry.

Probing subunit interactions in alpha class rat liver glutathione S-transferase with the photoaffinity label glutathionyl S-[4-(succinimidyl)benzophenone]

Glutathionyl S-[4-(succinimidyl)benzophenone] (GS-Succ-BP), an analogue of the product of glutathione and electrophilic substrate, acts as a photoaffinity label of dimeric rat liver glutathione S-transferase (GST), isoenzyme 1-1. A time-dependent loss of enzyme activity is observed upon irradiation of the enzyme with long wavelength UV light in the presence of the reagent. The initial rate of inactivation exhibits nonlinear dependence on the concentration of the reagent, characterized by an apparent dissociation constant of the enzyme-reagent complex (K(R)) of 99 +/- 2 microM and k(max) of 0.082 +/- 0.005 min(-1). Protection against this inactivation is provided by the electrophilic substrate (ethacrynic acid), electrophilic substrate analogue (dinitrophenol), and product analogues (S-hexylglutathione and p-nitrobenzylglutathione) but not by steroids (Delta(5)-androstene-3,17-dione and 17beta-estradiol-3, 17-disulfate). These results suggest that GS-Succ-BP binds and reacts with the enzyme within the xenobiotic substrate binding site, and this reaction site is distinct from the substrate and nonsubstrate steroid binding sites of the enzyme. About 1 mol of reagent is incorporated into 1 mol of enzyme dimer when the enzyme is completely inactivated. Met-208 is the only amino acid target of the reagent, and modification of this residue in one enzyme subunit of the GST 1-1 dimer completely abolishes the enzyme activity of both subunits. In order to evaluate the role of subunit interactions in the Alpha class glutathione S-transferases, inactive GS-Succ-BP-modified GST 1-1 was mixed with unlabeled, active GST 2-2. The enzyme subunits were dissociated in dilute trifluoroacetic acid and then renatured at pH 7.8 and separated by chromatofocusing into GST 1-1, 1-2, and 2-2. The specific activities of the heterodimer toward several substrates indicate that the loss of catalytic activity in the unmodified subunit of the modified GST 1-1 is the indirect result of the interaction between the two enzyme subunits and that this subunit interaction is absent in the heterodimer GST 1-2.

Homology between egg white sulfhydryl oxidase and quiescin Q6 defines a new class of flavin-linked sulfhydryl oxidases

The flavin-dependent sulfhydryl oxidase from chicken egg white catalyzes the oxidation of sulfhydryl groups to disulfides with the reduction of oxygen to hydrogen peroxide. Reduced proteins are the preferred thiol substrates of this secreted enzyme. The egg white oxidase shows an average 64% identity (from randomly distributed peptides comprising more than 30% of the protein sequence) to a human protein, Quiescin Q6, involved in growth regulation. Q6 is strongly expressed when fibroblasts enter reversible quiescence (Coppock, D. L., Cina-Poppe, D., Gilleran, S. (1998) Genomics 54, 460-468). A peptide antibody against Q6 cross-reacts with both the egg white enzyme and a flavin-linked sulfhydryl oxidase isolated from bovine semen. Sequence analyses show that the egg white oxidase joins human Q6, bone-derived growth factor, GEC-3 from guinea pig, and homologs found in a range of multicellular organisms as a member of a new protein family. These proteins are formed from the fusion of thioredoxin and ERV motifs. In contrast, the flavin-linked sulfhydryl oxidase from Aspergillus niger is related to the pyridine nucleotide-dependent disulfide oxidoreductases, and shows no detectable sequence similarity to this newly recognized protein family.