Inhibition of the reactivation of acid-dissociated lactate dehydrogenase isoenzymes by their aminoterminal CNBr fragments

The catalytic activity of lactate dehydrogenase isoenzymes (LDH) depends on their tetrameric structure. Stabilization of this quaternary structure is achieved by interaction of the N-terminal part of one subunit with the C-terminal region of the other subunit. The N-terminal peptides from pig M-LDH and H-LDH which are responsible for this stabilization were obtained by CNBr-fragmentation and purification on reversed-phase HPLC. The effect of these peptides on the formation of the quaternary structure of LDH-isoenzymes was investigated by monitoring the reconstitution of the catalytic activity after acid-dissociation. Low concentrations of the N-terminal peptides led to an increased, and high concentrations to a decreased yield of reconstituted LDH activity. The effects of these two peptides were isoenzyme specific. The 32 residue peptide derived from M-LDH showed the highest effect when tested with M-LDH as target enzyme but only a poor effect with H-LDH. On the other side the 33 residue peptide generated from H-LDH showed a moderate effect with both isoenzymes. The effects of the N-terminal LDH peptides are antagonized by the coenzymes NAD+ and NADH. The most significant influence was observed with NAD+ in the M-LDH peptide-M-LDH enzyme system. Comparison of the properties of the reactivation antagonists isolated from human origin with the N-terminal CNBr-peptides of LDH revealed identity in all essential properties, suggesting that the former peptides are generated by degradation of LDH.