Catalytic studies of glutathione transferase from Clarias gariepinus (Burchell) in dilute and crowded solutions

Molecular Cloning, Expression and Enzymatic Characterization of Tetrahymena thermophila Glutathione-S-Transferase Mu 34

Glutathione-S-transferase enzymes (GSTs) are essential components of the phase II detoxification system and protect organisms from oxidative stress induced by xenobiotics and harmful toxins such as 1-chloro-2,4-dinitrobenzene (CDNB). In Tetrahymena thermophila, the TtGSTm34 gene was previously reported to be one of the most responsive GST genes to CDNB treatment (LD50 = 0.079 mM). This study aimed to determine the kinetic features of recombinantly expressed and purified TtGSTm34 with CDNB and glutathione (GSH). TtGSTm34-8xHis was recombinantly produced in T. thermophila as a 25-kDa protein after the cloning of the 660-bp full-length ORF of TtGSTm34 into the pIGF-1 vector. A three-dimensional model of the TtGSTm34 protein constructed by the AlphaFold and PyMOL programs confirmed that it has structurally conserved and folded GST domains. The recombinant production of TtGSTm34-8xHis was confirmed by SDS‒PAGE and Western blot analysis. A dual-affinity chromatography strategy helped to purify TtGSTm34-8xHis approximately 3166-fold. The purified recombinant TtGSTm34-8xHis exhibited significantly high enzyme activity with CDNB (190 µmol/min/mg) as substrate. Enzyme kinetic analysis revealed Km values of 0.68 mM with GSH and 0.40 mM with CDNB as substrates, confirming its expected high affinity for CDNB. The optimum pH and temperature were determined to be 7.0 and 25 °C, respectively. Ethacrynic acid inhibited fully TtGSTm34-8xHis enzyme activity. These results imply that TtGSTm34 of T. thermophila plays a major role in the detoxification of xenobiotics, such as CDNB, as a first line of defense in aquatic protists against oxidative damage.

Induction and catalytic properties of grasshopper (Zonocerus variegatus) glutathione transferase fed on different food plants

In order to establish the role of diet on the induction and catalytic properties of glutathione transferase (GST) in insects, variegated grasshopper (Zonocerus variegatus) was exposed to different food plants separately for 30 days and the properties of the induced enzyme were then investigated. Insects fed on cassava (M. esculenta) leaves had the highest GST induction followed by insects fed on bitter leaf (V. amygdalina). Z. variegatus that fed in the wild on different food plants had the least suggesting that allelochemicals in the food plants have a compensatory toxicity-alleviating actions on one another. 1-Chloro-2,4-dinitrobenzene (CDNB) was the best substrate for all the induced GST however, the mode of binding of the substrate to the induced enzyme was not the same. GST from M. esculenta-fed insect showed ping-pong kinetic mechanism whereas GSTs from V. amygdalina and T. procumbens-fed insects showed random sequential mode of substrate binding. Catalytic efficiency (k_cat/K_m) of GST from M. esculenta-fed insects was 3-8-fold higher than other induced enzymes. Commercial insecticides- cypermethrin and lindane had an inhibition constant, K_i, of 0.13±0.004 mM and 0.68±0.09 mM, respectively, suggesting that the concentration as used in the field (0.03 mM for cypermethrin and 0.3 mM for lindane) would have little effect on the insect's GST. The study concluded that higher GST activity are induced in insects that fed on monotonous diets than those that fed on various food plants. Hindgut appears to be the primary organ of detoxication. The catalytic properties of the induced enzymes are different from one another.