NAD+-dependent ethanol oxidation: redox effects and rate limitation

Area of Science:

• Biochemistry
• Cellular Metabolism
• Enzymology

Background:

• Ethanol metabolism involves redox reactions crucial for cellular energy balance.
• Alcohol dehydrogenase (ADH) plays a key role in ethanol oxidation, utilizing the NAD+/NADH cofactor system.
• Understanding cofactor dynamics is essential for elucidating metabolic regulation.

Purpose of the Study:

• To investigate the effects of ethanol on the interconversion of cyclohexanol and cyclohexanone in isolated hepatocytes.
• To determine how ethanol oxidation impacts the NAD+/NADH ratio bound to alcohol dehydrogenase.
• To assess the equilibration of coenzymes within cytosolic redox reactions during ethanol metabolism.

Main Methods:

• Studied the oxidation and reduction of cyclohexanol/cyclohexanone catalyzed by alcohol dehydrogenase in isolated hepatocytes.
• Utilized deuterium-labeled ethanol ([1,1-2H2] ethanol) and glycerol ([2-2H] glycerol) to trace coenzyme transfer.
• Measured the incorporation of deuterium into metabolic products to infer enzyme activity and coenzyme status.

Main Results:

• Ethanol markedly inhibited ADH-catalyzed oxidation and stimulated reduction.
• The ratio of NAD+ to NADH bound to ADH decreased significantly, indicating a shift towards NADH.
• Deuterium incorporation patterns suggested that bound coenzymes are not fully equilibrated with free coenzymes, and NADH dissociation may be rate-limiting for ethanol oxidation.
• Cytosolic malate dehydrogenase and lactate dehydrogenase showed incomplete equilibration, while NADH utilization from ADH and glycerol-3-phosphate dehydrogenase was similar.

Conclusions:

• Ethanol oxidation leads to a substantial decrease in the NAD+/NADH ratio bound to ADH.
• The dissociation of NADH from ADH may be a rate-limiting step in ethanol metabolism.
• High redox reaction fluxes during ethanol oxidation may prevent complete equilibration of cytosolic dehydrogenases.