Affinity labeling of pig lung glutathione S-transferase pi by 4-(fluorosulfonyl)benzoic acid

Sulfonyl fluorides as privileged warheads in chemical biology

The use of sulfonyl fluoride probes in chemical biology is reviewed.

Sulfonyl fluoride electrophiles have found significant utility as reactive probes in chemical biology and molecular pharmacology. As warheads they possess the right balance of biocompatibility (including aqueous stability) and protein reactivity. Their functionality is privileged in this regard as they are known to modify not only reactive serines (resulting in their common use as protease inhibitors), but also context-specific threonine, lysine, tyrosine, cysteine and histidine residues. This review describes the application of sulfonyl fluoride probes across various areas of research and explores new approaches that could further enhance the chemical biology toolkit. We believe that sulfonyl fluoride probes will find greater utility in areas such as covalent enzyme inhibition, target identification and validation, and the mapping of enzyme binding sites, substrates and protein–protein interactions.

Mice lacking three Loci encoding 14 glutathione transferase genes: a novel tool for assigning function to the GSTP, GSTM, and GSTT families

Glutathione S-transferases (GSTs) form a superfamily defined by their ability to catalyze the conjugation of glutathione with electrophilic substrates. These enzymes are proposed to play a critical role in protection of cellular components from damage mediated by reactive metabolites. Twenty-two cytosolic GSTs, grouped into seven families, are recognized in mice. This complexity hinders the assignment of function to a subset or family of these genes. We report generation of a mouse line in which the locus encoding three GST gene families is deleted. This includes the four Gstt genes spanning 65 kb on chromosome 10 and the seven Gstm genes found on a 150 kb segment of DNA chromosome 3. In addition, we delete two Gstp genes on chromosome 19 as well as a third related gene located 15 kb telomeric to Gstp1 and Gstp2, which we identify as a potential new member of this gene family. We show that, despite the loss of up to 75% of total GST activity in some tissues from these animals, the mice are healthy and fertile, with normal life expectancy. The normal development and health of these animals make them an appropriate model for defining the role of these families in redox homeostasis and metabolism of drugs and environmental pollutants.

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.

Glutathione conjugation as a bioactivation reaction

In general, glutathione conjugation is regarded as a detoxication reaction. However, depending on the properties of the substrate, bioactivation is also possible. Four types of activation reaction have been recognized: direct-acting compounds, conjugates that are activated through cysteine conjugate beta-lyase, conjugates that are activated through redox cycling and lastly conjugates that release the original reactive parent compound. The glutathione S-transferases have three connections with the formation of biactivated conjugates: they catalyze their formation in a number of cases, they are the earliest available target for covalent binding by these conjugates and lastly, the parent alkylating agents are regularly involved in the induction of the enzymes. Individual susceptibility for each of these agents is determined by individual transferase subunit composition and methods are becoming available to assess this susceptibility.