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Characterization of apparent lactate dehydrogenase isoenzyme 6: a lactate-independent dehydrogenase
Insights
Lactate dehydrogenase isoenzyme 6 (LD6) appears in severe liver injury, particularly after prolonged hypotension or poor ventilation. This heat-stable enzyme, found in liver, kidney, and spleen, is not a true lactate dehydrogenase.
Area of Science:
- Biochemistry
- Clinical Pathology
- Enzymology
Background:
- Lactate dehydrogenase (LD) is a key enzyme in cellular metabolism.
- LD exists as isoenzymes (LD1-LD5) composed of H and M subunits.
- The presence and significance of other LD isoenzymes, like LD6, remain less understood.
Purpose of the Study:
- To investigate the presence and characteristics of lactate dehydrogenase isoenzyme 6 (LD6) in human tissues.
- To determine the conditions associated with LD6 presence.
- To elucidate the biochemical nature of LD6.
Main Methods:
- Prospective autopsy study of human tissues.
- Analysis of LD6 presence in liver, kidney, and spleen.
- Biochemical characterization including heat stability, pyruvate resistance, immunoprecipitation, and substrate analysis.
Main Results:
- LD6 was frequently detected in liver tissue following prolonged hypotension or impaired ventilation, but not in cases of sudden death.
- LD6 was also found in kidney and spleen tissues.
- Biochemical assays indicated LD6 is heat stable, distinct from other LD isoenzymes, and functions without lactate as a substrate, being enhanced by ethanol.
Conclusions:
- LD6 is not a true lactate dehydrogenase.
- The presence of LD6 in liver is indicative of severe liver injury, often associated with hypoxic conditions.
- Further research is warranted to fully understand the role and origin of LD6.
Abstract:
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.