Subunit cooperativity in the action of lactate dehydrogenase

Abstract

Interspecific hybrids of pig heart lactate dehydrogenase ( l-lactate:NAD + oxidoreductase, EC 1.1.1.27) isozyme((PH) 4) and chicken heart lactate dehydrogenase isozyme((CH) 4) were obtained by freezing and thawing. Heat inactivation profiles of the interspecific hybrids revealed: 1. 1.|At 70°, the pig heart lactate dehydrogenase ((PH) 4) is appreciably less stable than the chicken heart lactate dehydrogenase ((CH) 4). 2. 2.|The hybrid isozyme ((CH) 3(PH) 1) is completely inactivated along with the inactivation of the pig heart isozyme ((PH) 4). To explain these observations, four types of models (“active monomer” model, “active dimer” model, “active trimer” model and “active tetramer” model) were tested. The theoretical heat inactivation profiles deduced from the “active dimer” model gives the best fit to the experimental heat inactivation profiles. This fact suggests that two protomers act in cooperation as an active dimer in a tetrameric molecule of lactate dehydrogenase.