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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.

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The degree of substrate inhibition of acetylcholine esterase activity decreases in plasma synaptosome membranes isolated from the cerebral cortex of rats subjected to short-term and prolonged hypothermia at 20~ while activation energy and breakpoint of the temperature curve in the Arrhenius coordinates remain unaffected. The observed changes are supposed to compensate the inhibitory effect of hypothermia on enzyme activity. Key Words: acetylcholine esterase; temperature dependence," synaptic membranes," hypothermia, ratsEven a short-term drop of core temperature to 20~ in homeothermic animals induces adaptive changes in the enzymatic apparatus in the brain [3]. A peak appears on the temperature plot of activity of the glutaminase isolated from synaptosome brain fraction, which corresponds to body temperature and suggests activation of temperature compensation of synaptic transmission, because glutaminase catalyzes synthesis of excitatory transmitter glutamate in glutamatergic synapses. The same changes probably take place in other neurotransmitter cerebral systems during hypothermia. Acetylcholine esterase (ACE) is a key enzyme in cholinergic cerebral synapses. Here we studied the temperature dependence of activity of this enzyme in rats subjected to 20~ hypothermia. MATERIALS AND METHODSThe study was carried out on 54 random-bred male albino rats weighing 170-200 g. The animals were divided into three groups. Group 1 rats were control, group 2 rats were subjected to 20~ hypothermia, and group 3 rats were subjected to a long-term hypothermia