Antioxidant Status of Rat Liver Mitochondria under Conditions of Moderate Hypothermia of Different Duration.

For evaluation of the contribution of the antioxidant system of mitochondria into the dynamics of changes in the prooxidant status, the content and activity of some of its components were studied under conditions of moderate hypothermia of varying duration. It was found that short-term hypothermia significantly increased superoxide dismutase activity and decreased the levels of low-molecular-weight antioxidants. Increasing the duration of hypothermia to 1 h led to suppression of activities of superoxide dismutase, glutathione reductase, and glutathione peroxidase and a decrease in glutathione content. Further prolongation of hypothermia (to 3 h) was associated with a significant increase in superoxide dismutase and glutathione peroxidase activities and normalization of the rate of glutathione reductase catalysis; the concentration of glutathione increased significantly.

Respiratory Characteristics of Rat Liver Mitochondria Depend on the Duration of Moderate Hypothermia.

The development of pathological or compensatory-and-adaptive reactions in homoeothermic animals during various periods of hypothermia can be caused by shifts in the respiratory functions of the mitochondria. Short-term hypothermia promoted an increase in the rates of the glutamate- and succinate-dependent respiration of mitochondria. Phosphorylation rate increased as well, while oxidative phosphorylation coefficient (P/O), respiratory control, and 2,4-DNP sensitivity depended. Changes in respiratory characteristics in the dynamics of prolonged hypothermia depends on the type of substrate. Prolongation of hypothermia to 1 h was associated with further intensification of succinate-dependent respiration, decrease in P/O and respiratory control, while prolongation of hypothermia to 3 h led to their normalization. The majority of respiratory characteristics of glutamate-dependent respiration did not change under these conditions and their levels were the same as during short-term hypothermia.

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.