The African clawed frog, Xenopus laevis endures whole body
dehydration which can increase its reliance on anaerobic glycolysis for energy production. This makes the regulation of the terminal
enzyme of glycolysis,
lactate dehydrogenase (LDH), crucial to stress survival. We investigated the enzymatic properties and posttranslational modification state of purified LDH from the skeletal muscle of control and dehydrated (30% total body water loss) X. laevis. LDH from the muscle of dehydrated frogs showed a 93% reduction in phosphorylation on
threonine residues and an 80% reduction of
protein nitrosylation. LDH from dehydrated muscle also showed a 74% lower Vmax in the
pyruvate oxidizing direction and a 78% decrease in Vmax in the
lactate reducing direction along with a 33% lower Km for
pyruvate and a 40% higher Km for
lactate. In the presence of higher levels of
urea and molecular crowding by
polyethylene glycol, used to mimic conditions in the cells of dehydrated animals, the Km values of control and dehydrated LDH demonstrated opposite responses. In the
pyruvate oxidizing direction, control muscle LDH was unaffected by these additives, whereas the affinity for
pyruvate dropped further for LDH from dehydrated muscle. The opposite effect was more pronounced in the
lactate reducing direction as control LDH showed an increased affinity for
lactate, whereas LDH from dehydrated animals showed a further reduction in affinity. The physiological consequences of
dehydration-induced LDH regulation appear to poise the
enzyme towards
lactate production when
urea levels are high and
lactate catabolism when
urea levels are low, perhaps helping to maintain glycolysis under dehydrating conditions whilst providing for the ability to recycle
lactate upon
rehydration.