Stability of lactate dehydrogenase. II. Hybrids and geometric isomers

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 (HP2MP2 and HC2HP2) 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 characteristics temperatures of denaturation were 61.5 degrees C for HP4, HCHP3 and HC2HP2I; 71 degrees C for HC2HP2II and HC3HP and 76.5 degrees for HC4. 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 or two Q-contacts of the tetramer.