Thermostability of Lactate Dehydrogenase in Rat Brain under Conditions of Short-Term Moderate Hypothermia

Thermostability of rat brain lactate dehydrogenase (LDH) was studied in intact animals and animals subjected to moderate short-term hypothermia. Two exponential stages, rapid and slow, were distinguished in the thermodenaturation kinetics. The contribution of the rapid phase to the lactate dehydrogenase denaturation kinetics was more significant: the energy of activation for this phase was 2.33 times lower than that for the slow phase. Moderate shortterm hypothermia led to a significant decrease of lactate dehydrogenase thermostability: thermodenaturation rate constants for the rapid (k₁) and slow (k₂) phases increased. Significant changes in parameters a and b reflecting the initial proportion of the two native forms of the enzyme developed only at 40°C. As hypothermia caused no appreciable changes in the energy of activation of lactate dehydrogenase denaturation, a significant contribution of the entropic factor to the decrease of free energy of enzyme denaturation was hypothesized. The data indicated significant labilization of lactate dehydrogenase structure under conditions of moderate hypothermia.

Effect of Hypothermia on Kinetic Characteristics of Lactate Dehydrogenase in Rat Brain under Conditions of Global Ischemia and Reperfusion

We studied activity and kinetic characteristics of lactate dehydrogenase (LDH) in rat brain under conditions of incomplete global ischemia followed by reperfusion against the background of mild hypothermia. It was found that hypothermia leads to a decrease in LDH activity in the ischemic brain; the maximum velocity of the enzyme-catalyzed activity decreased and Michaelis constant increased, due to which the efficiency of catalysis decreased to the level observed in control rats. Ischemia against the background of hypothermia was accompanied by a decrease in the inhibition constant and narrowing of effective pyruvate concentration range. Blood flow resumption in the ischemic brain against the background of mild hypothermia led to an increase in LDH activity, the maximum reaction velocity increased, and Michaelis constant decreased, which lead to a significant increase in the efficiency of catalysis. This was accompanied by an increase in enzyme inhibition constant and a shift of the optimum on the concentration curve towards lower pyruvate concentrations.