A Comparison of Lactate Dehydrogenases from an Ectothermic and an Endothermic Animal

Abstract

Adaptation to environmental temperature is examined in beef heart, beef muscle, and flounder muscle lactate dehydrogenases (EC 1.1.1.27). Low temperature adaptation in the ectothermic (flounder) enzyme is indicated by a reduced enthalpy of activation for k_cat (enzyme turnover number, s⁻¹) and increased catalytic efficiency. Also, the reaction rate at low substrate concentrations has a maximum at a lower temperature than in the endothermic enzymes. This is a result of altered bonding in the enzyme–substrate complexes. Adaptation to higher temperatures in the endothermic (beef) enzymes is suggested by a decreased sensitivity to heat denaturation, especially in the presence of substrates. A direct correlation is found between the degree of bonding in the enzyme–substrate complexes and the decrease in rate of heat denaturation caused by addition of substrates.