Effect of guanidination on subunit interactions in hybrid isozymes from pig lactate dehydrogenase

The thermostability of the isozymes from pig heart (H) and muscle (M) lactate dehydrogenase (L-lactate:NAD+ oxidoreductase, EC 1.1.1.27) decreased in the order of H4 greater than M4 greater than H3M greater than HM3 greater than H2M2, while the thermostability of the isozymes from guanidinated H4 and M4 increased as guanidinated H monomer was substituted by M monomer. The increased thermostability of H4 increased as guanidinated H monomer was substituted by M monomer. The increased thermostability of H4 on guanidination of five lysine residues per subunit was due to the decrease in the standard activation entropy, and no change in the standard activation enthalpy was observed. The more increased thermostability of H4 on further guanidination of lysine residues from 5 to 15 per subunit was due to the increase of the standard activation enthalpy which overcame the decrease in thermostability due to the increase of the standard activation entropy. The results indicate two different mechanisms of stabilization depending on the degree of guanidination. The increase of thermostability, as measured by the change of the standard activation free energy for thermal inactivation of H2M2, was almost the same as that of H4 on guanidination of five lysine residues in an H monomer. This result and the order of thermostability of the isozymes from unmodified and guanidinated H4 and M4 suggest that the increase of thermostability of hybrid isozymes on guanidination of H monomer is due to the change of the heterologous subunit interactions.