Effect of temperature upon catalytic properties of lactate dehydrogenase isoenzymes from a poikilotherm

Lactate dehydrogenase isoenzymes A4 and B4 (l-lactate:NAD+ oxidoreductase, EC 1.1.1.27) have been purified from muscle and heartof the snake Bothrops neuwiedii. Catalytic properties at different temperatures and pH values have been studied compartively with those of the same isoenzymes from beef. Values of Km for ophidian isoenzymes were markedly reduced when the temperature decreased. At low substrate concentrations, the increased enzyme-pyruvate affinity compensates for the lower thermal energy and the reaction rate appears to be independent of temperature. Under the same conditions, the activity of beef isoenzymes was closely related to thermal energy. These findings suggest that lactate dehydrogenases from the poikilotherm are able to function with the same efficiency at all temperatures within the range of habitat and to adjust immediately to thermal changes.