Lactate dehydrogenase (LDH) is the terminal
enzyme of anaerobic glycolysis and key to
hypoxia/
anoxia survival by most animals. In this study, the effects of anoxic submergence (20 h at 7°C in
nitrogen-bubbled water) were assessed on LDH from liver of an
anoxia-tolerant freshwater turtle, the red-eared slider (Trachemys scripta elegans). Liver LDH from aerobic and anoxic turtles was purified to homogeneity in two steps. The kinetic properties and thermal stability of purified LDH were analyzed, revealing significant differences between the two
enzyme forms in V(max), K(m)
pyruvate, and I(50)
pyruvate as well as melting temperature determined by differential scanning fluorimetry. The phosphorylation state of aerobic and anoxic forms of LDH was visualized by ProQ
Diamond phosphoprotein staining, the results indicating that the anoxic form had a higher phosphorylation state. Incubation studies that promoted
protein kinase versus
protein phosphatase actions showed that changes in the phosphorylation state of aerobic and anoxic forms mimicked the
anoxia-responsive changes in K(m)
pyruvate and I(50)
pyruvate. The high
phosphate form of liver LDH that occurs in anoxic turtles appears to be a less active form. Turtle liver LDH was also subject to another form of posttranslational modification,
protein acetylation, with a 70% higher content of acetylated
lysine residues on anoxic versus aerobic LDH. This is the first study to show that LDH function in an
anoxia-tolerant animal can be differentially modified between aerobic and anoxic states via the mechanism of posttranslational modification.