Regulation of glutathione-based antioxidant defenses in response to dehydration stress in the African Clawed frog, Xenopus laevis
The African clawed frog (Xenopus laevis) needs efficient antioxidant defenses to mitigate oxidative damage and endure dehydration stress under arid conditions in its natural environment. Two enzymes that aid glutathione-based antioxidant defenses, glutathione reductase (GR) and glucose-6-phosphate dehydrogenase (G6PDH), were purified from liver of control and dehydrated (~35% total body water lost) frogs. Kinetic analysis revealed that GR was positively regulated in response to dehydration to regenerate GSH, particularly when physiological urea concentrations were high. G6PDH from dehydrated frogs showed lower affinity for glucose-6-phosphate than control but was strongly activated in the presence of high ATP. Both enzymes showed regulatory modification by reversible protein phosphorylation that affected substrate affinities. Dehydration also induced other PTMs including protein sumoylation, cys-nitrosylation, and acetylation that could affect kinetic properties and protein stability. This thesis provides novel insights into the regulation of glutathione-based enzymatic defenses against dehydration-induced oxidative stress.