Purification and characterization of a urea sensitive lactate dehydrogenase from the liver of the African clawed frog, Xenopus laevis
The African clawed frog, Xenopus laevis, is able to withstand extremely arid conditions by estivating, in conjunction with dehydration tolerance and urea accumulation. Estivating X. laevis reduce their metabolic rate and recruit anaerobic glycolysis, driven by lactate dehydrogenase (LDH; E.C. 1.1.1.27) enzymes that reversibly convert pyruvate and NADH to lactate and NAD+, to meet newly established ATP demands. The present study investigated purified LDH from the liver of dehydrated and control X. laevis. LDH from dehydrated liver showed a significantly higher Km for l-lactate (1.74 fold), NAD+ (2.41 fold), and pyruvate (1.78 fold) in comparison to LDH from the liver of control frogs. In the presence of physiological levels of urea found in dehydrated animals, the Km values obtained for dehydrated LDH all returned to control LDH Km values. Dot blot analysis showed post-translational modifications may be responsible for the kinetic modification as the dehydrated form of LDH showed more phosphorylated serine residues (1.54 fold), less methylated lysine residues (0.43 fold), and a higher level of ubiquitination (1.90 fold) in comparison to control LDH. The physiological consequence of dehydration-induced LDH modification appears to adjust LDH function in conjunction with urea levels in dehydrated frogs. When urea levels are high during dehydration, LDH retains its normal function. Yet, as urea levels drop during rehydration, LDH function is reduced, possibly shunting pyruvate to the TCA cycle.