Purification of rat liver plasma membrane glutathione transferase

A role for microsomal glutathione transferase 1 in melanin biosynthesis and melanoma progression

Recent advancements in the treatment of melanoma are encouraging, but there remains a need to identify additional therapeutic targets. We identify a role for microsomal glutathione transferase 1 (MGST1) in biosynthetic pathways for melanin and as a determinant of tumor progression. Knockdown (KD) of MGST1 depleted midline-localized, pigmented melanocytes in zebrafish embryos, while in both mouse and human melanoma cells, loss of MGST1 resulted in a catalytically dependent, quantitative, and linear depigmentation, associated with diminished conversion of L-dopa to dopachrome (eumelanin precursor). Melanin, especially eumelanin, has antioxidant properties, and MGST1 KD melanoma cells are under higher oxidative stress, with increased reactive oxygen species, decreased antioxidant capacities, reduced energy metabolism and ATP production, and lower proliferation rates in 3D culture. In mice, when compared to nontarget control, Mgst1 KD B16 cells had less melanin, more active CD8+ T cell infiltration, slower growing tumors, and enhanced animal survival. Thus, MGST1 is an integral enzyme in melanin synthesis and its inhibition adversely influences tumor growth.

Microsomal glutathione transferase 1 in cancer and the regulation of ferroptosis

Microsomal glutathione transferase 1 (MGST1) is a member of the MAPEG family (membrane associated proteins in eicosanoid and glutathione metabolism), defined according to enzymatic activities, sequence motifs, and structural properties. MGST1 is a homotrimer which can bind three molecules of glutathione (GSH), with one modified to a thiolate anion displaying one-third-of-sites-reactivity. MGST1 has both glutathione transferase and peroxidase activities. Each is based on stabilizing the GSH thiolate in the same active site. MGST1 is abundant in the liver and displays a broad subcellular distribution with high levels in endoplasmic reticulum and mitochondrial membranes, consistent with a physiological role in protection from reactive electrophilic intermediates and oxidative stress. In this review paper, we particularly focus on recent advances made in understanding MGST1 activation, induction, broad subcellular distribution, and the role of MGST1 in apoptosis, ferroptosis, cancer progression, and therapeutic responses.

Enzymology of reactive intermediate protection: kinetic analysis and temperature dependence of the mesophilic membrane protein catalyst MGST1

Glutathione transferases (GSTs) are a class of phase II detoxifying enzymes catalysing the conjugation of glutathione (GSH) to endogenous and exogenous electrophilic molecules, with microsomal glutathione transferase 1 (MGST1) being one of its key members. MGST1 forms a homotrimer displaying third-of-the-sites-reactivity and up to 30-fold activation through modification of its Cys-49 residue. It has been shown that the steady-state behaviour of the enzyme at 5 °C can be accounted for by its pre-steady-state behaviour if the presence of a natively activated subpopulation (~ 10%) is assumed. Low temperature was used as the ligand-free enzyme is unstable at higher temperatures. Here, we overcame enzyme lability through stop-flow limited turnover analysis, whereby kinetic parameters at 30 °C were obtained. The acquired data are more physiologically relevant and enable confirmation of the previously established enzyme mechanism (at 5 °C), yielding parameters relevant for in vivo modelling. Interestingly, the kinetic parameter defining toxicant metabolism, k_cat /K_M , is strongly dependent on substrate reactivity (Hammett value 4.2), underscoring that glutathione transferases function as efficient and responsive interception catalysts. The temperature behaviour of the enzyme was also analysed. Both the K_M and K_D values decreased with increasing temperature, while the chemical step k₃ displayed modest temperature dependence (Q₁₀ : 1.1-1.2), mirrored in that of the nonenzymatic reaction (Q₁₀ : 1.1-1.7). Unusually high Q₁₀ values for GSH thiolate anion formation (k₂ : 3.9), k_cat (2.7-5.6) and k_cat /K_M (3.4-5.9) support that large structural transitions govern GSH binding and deprotonation, which limits steady-state catalysis.

The odorant metabolizing enzyme UGT2A1: Immunolocalization and impact of the modulation of its activity on the olfactory response

Odorant metabolizing enzymes (OMEs) are expressed in the olfactory epithelium (OE) where they play a significant role in the peripheral olfactory process by catalyzing the fast biotransformation of odorants leading either to their elimination or to the synthesis of new odorant stimuli. The large family of OMEs gathers different classes which interact with a myriad of odorants alike and complementary to olfactory receptors. Thus, it is necessary to increase our knowledge on OMEs to better understand their function in the physiological process of olfaction. This study focused on a major olfactory UDP-glucuronosyltransferase (UGT): UGT2A1. Immunohistochemistry and immunogold electronic microscopy allowed to localize its expression in the apical part of the sustentacular cells and originally at the plasma membrane of the olfactory cilia of the olfactory sensory neurons, both locations in close vicinity with olfactory receptors. Moreover, using electroolfactogram, we showed that a treatment of the OE with beta-glucuronidase, an enzyme which counterbalance the UGTs activity, increased the response to eugenol which is a strong odorant UGT substrate. Altogether, the results supported the function of the olfactory UGTs in the vertebrate olfactory perireceptor process.

Microsomal glutathione transferase 1: mechanism and functional roles

Microsomal glutathione transferase 1 (MGST1) belongs to a superfamily named MAPEG (membrane-associated proteins in eicosanoid and glutathione metabolism). This family is represented in all life forms, except archae. Of the six human members, three are specialized in the synthesis of leukotrienes and prostaglandin E, whereas the others (MGST1-3) have potential roles in drug metabolism. MGST1 has a well-established role in the conjugation of electrophiles and oxidative stress protection, whereas MGST2 and 3 have been less studied. Here, we review the recent advances regarding the structure, mechanism, and functional roles of MGST1. Emerging data show that the enzyme is overexpressed in certain tumors and support a role for the enzyme in protecting cells from cytostatic drugs.

Sperm plasma-membrane-associated glutathione S-transferases as gamete recognition molecules

Glutathione S-transferases (GSTs) are enzymes that detoxify electrophilic compounds. Earlier studies from our laboratory showed that anti-GST antibodies interfered with the fertilising ability of spermatozoa from Capra hircus (goat) in vitro, suggesting that GSTs are localised at the cell surface. In this study, we provide evidence for the presence of GSTs of 24 kDa on the sperm plasma membrane attached by non-covalent interactions. The GST activity associated with the spermatozoal plasma membrane was significantly higher than the activity present in the plasma membranes of brain cells,hepatocytes, spleenocytes and ventriculocytes. Analysis of GST isoforms demonstrates the presence of GST Pi and Mu on the sperm plasma membranes. Both isoforms were able to bind to solubilised as well as intact zona pellucida(ZP) through their N-terminal regions but failed to bind to ZP once the oocytes were fertilised. Solubilised goat ZP separates into three components,one of which, the ZP3-like component, bound to sperm GSTs. High concentrations of anti-GST antibodies or solubilised ZP led to aggregation of sperm GSTs,resulting in the release of acrosin. In contrast, inhibition of sperm GST binding to ZP, by saturation of binding sites for sperm GSTs on the solubilised ZP using peptides designed from the N-terminii of GST Pi or Mu or blocking of binding sites for ZP on sperm GSTs with antibodies raised against the N-terminal GST peptides, inhibited essential prefertilisation changes in sperm.

These data therefore demonstrate the strategic location of catalytically active defensive enzymes on the sperm surface that also act as zona-binding proteins. Therefore, sperm-surface GSTs serve as bifunctional molecules in a transcriptionally inactive cell whose requirement for cellular defense and economy of molecules that it can carry is greater than that of any somatic cell type.

Multiple glutathione S-transferase isoforms are present on male germ cell plasma membrane

Phase II detoxification enzymes, the glutathione S-transferases (GSTs) of 24 kDa are known to be cytosolic enzymes. This study shows that multiple GST isoforms that are 24 kDa in size are present on the extracellular side of the plasma membrane of rat male germ cells. The GST activity of male germ cell plasma membranes is several folds higher than somatic cell plasma membrane GST activity. Isoform composition of the germ cell plasma membrane and the cytosolic pool differ, GSTM5 and GSTPi being absent on the plasma membranes. The molecular masses of the common isoforms are comparable between the two pools and both pools show GST and glutathione peroxidase activity.

Protein S-thiolation and redox regulation of membrane-bound glutathione transferase

Membrane-bound GST transferase (GSTm) occurs in hepatic microsomal and plasma membranes as well as in the outer mitochondrial membrane, and it is known to be activated by N-ethylmaleimide. We recently analysed the activation by GSSG in some detail. The approximately 5-fold stimulation is reversed upon reduction of GSSG by GSSG reductase. In steady-state experiments, the Kox value was determined to be 0.05, i.e. 20 times more GSSG than GSH produces half-maximal activation. Kox is independent of the total glutathione concentration, indicating that S-thiolation by mixed disulfide formation, rather than interchain or intrachain disulfide bridge formation, is responsible for activation. In Western blots, a 17.7 kDa band, in addition to the 17.3 kDa band, was detected upon treatment with GSSG or with GSH plus t-butyl hydroperoxide. We suggest that under oxidative stress, GSTm is activated through direct S-thiolation of the enzyme. Dethiolation occurs via thiol disulfide exchange governed by the cellular glutathione redox state.

Microsomal formation of S-nitrosoglutathione from organic nitrites: possible role of membrane-bound glutathione transferase

The formation of S-nitrosoglutathione (GSNO) from amyl nitrite and n-butyl nitrite was studied in rat liver microsomes, employing N-ethylmaleimide (MalNEt) as an activator and indomethacin as an inhibitor of microsomal glutathione S-transferase (GST). Rates were compared with GST activity measured with 1-chloro-2,4-dinitrobenzene (CDNB) as a substrate. MalNEt stimulated GST activity and the formation of GSNO from amyl nitrite and n-butyl nitrite about 10-fold. Increasing concentrations of indomethacin inhibited both reactions in parallel. N-Acetyl-L-cysteine but not L-cysteine could substitute for GSH. It is concluded that rat liver microsomal GST catalyses the formation of GSNO from amyl nitrite and n-butyl nitrite. The activity of the MalNEt-stimulated microsomal GST is calculated to be about 17 units/mg of enzyme with the alkyl nitrites and about 16 units/mg of enzyme with CDNB as a substrate, assuming that 3% of microsomal protein is GST. These rates are comparable with those obtained for cytosolic GSTs. Thus microsomal GST may play a significant role in the metabolism of alkyl nitrites in biological membranes.