Stability of lactate dehydrogenase

Hybrids of lactate dehydrogenases from pig heart and muscle and from chicken heart and pig heart were obtained by the freeze-thaw method [1,2]. Ion-exchange chromatography of the resulting mixtures of hybrids yielded unusual elution patterns, i.e., the 2 + 2 hybrids (H₂^PM₂^P and H₂^CH₂^P) were eluted in two separate peaks. These subforms were concluded to result from partial resolution of the three geometric isomers. The hybrids of chicken heart and pig heart lactate dehydrogenase showed three distinct levels of stability. The characteristic temperatures of denaturation were 61.5°C for H₄^P, H^CH₃^P and H₂^CH₂^PI; 71°C for H₂^CH₂^PII and H₃^CH^P and 76.5°C for H₄^C. The resistance towards thermal denaturation thus seemed to be governed by the least stable dimer within the tetrameric enzyme. The arrangement of stabilities of the dimers was in excellent agreement with the number of additional ion pairs between Arg₂₄₁ (chicken) and Asp₅₇ (chicken and pig) [3] within the Q-contact areas. The rate-determining step of thermal denaturation of lactate dehydrogenase was concluded to comprise the distortion or dissociation of one of two Q-contacts of the tetramer.