Stability of lactate dehydrogenases. I. Chemical modification of lysines

The lysine residues of lactate dehydrogenase (L-lactate: NAD+ oxidoreductase, EC 1.1.1.27) can be amidinated by methyl-4-hydroxy-3-nitrobenzimidate to introduce nitrophenolate anions. This modification results in lowered thermal stability, as does acetylation. The conversion of these groups into uncharged aminophenol groups without further modification of the enzyme itself stabilizes the enzymes from pig heart and muscle and from chicken muscle, as does acetamidination, but the unusually stable enzyme from chicken heart reverts only to the stability of the native form. The results allow for the following conclusions. Destabilization is brought about at many points at the surface of lactate dehydrogenases by neutralization of positive charges. Stabilization, in contrast, is concluded to be due to modification of one lysine at position 241 of the sequence. This lysine must have been changed to arginine during the evolution of heart-type lactate dehydrogenases in going from lower to higher reptiles. This exchange has been conserved in the enzymes from the hearts of birds and therefore the enzyme from chicken heart is very stable and cannot further be stabilized by modification of lysines. From X-ray structure analysis, the stabilization by exchange of Arg for Lys at position 241 or by amidination is explained by the formation of additional ion pairs with aspartic acid57 of the Q-related subunits.