Characterization of apparent lactate dehydrogenase isoenzyme 6: a lactate-independent dehydrogenase.

We found in a prospective study of lactate dehydrogenase isoenzyme 6 (LD6) in human tissues obtained at autopsy that LD6 was usually present in liver when death was preceded by prolonged hypotension or impaired ventilation but not in cases of sudden death. Other organs containing LD6 were kidney and spleen. LD6 is heat stable, differs from H-subunit-containing LD isoenzymes by pyruvate resistance, differs from M-subunit-containing LD isoenzymes by immunoprecipitation, and is distinct from spermatic LDX. LD6 from liver extracts acted without lactate as substrate and could be enhanced by ethanol added to the substrate. These results indicate that LD6 is not a true lactate dehydrogenase, and that it frequently appears in severe liver injury.