I’ve been digging into a few studies on alcohol dehydrogenase — the enzyme responsible for the first step of ethanol metabolism, oxidizing ethanol to acetaldehyde while reducing NAD+ to NADH. I found some genuinely strange toxicology, and some insight into hangovers.
When It’s Not Ethanol: A Forensic Toxicology Case
Ethanol is not the only substrate that alcohol dehydrogenase can oxidize. Jones (2024) analyzed blood samples from people arrested for drunk driving and found something unexpected: acetone levels as high as 97 mg/L in some samples. That’s notable because alcohol dehydrogenase also oxidizes isopropanol (rubbing alcohol) into acetone — so elevated acetone is often a sign that someone drank something other than a standard alcoholic beverage.
In this case, researchers confirmed the individuals had consumed industrial-grade alcohol rather than rubbing alcohol — denatured alcohol, with acetone at a concentration of 2–5% as its denaturing agent.
The samples also showed elevated methanol — 48 mg/L, well above the 0.5–1.5 mg/L found to be produced endogenously in other studies they cited. Since methanol wasn’t a component of the product ingested, the researchers proposed that this was a buildup of the body’s own naturally-produced methanol rather than from an external source. The mechanism comes down to competition: alcohol dehydrogenase also oxidizes methanol, but it strongly prefers ethanol — by a factor of ten. They cited other research where it was found that methanol metabolism doesn’t really resume until blood ethanol drops below 100–200 mg/L, after which methanol has a half-life of 2–3 hours.
That creates a slow, staged clearance process. As blood ethanol falls, methanol metabolism gradually kicks back in; in this case having to work its way down from 48 mg/L to about 1 mg/L, one half-life at a time — with methanol’s toxic byproducts, formaldehyde and formic acid, being generated throughout. For someone with chronic heavy alcohol use, that extended clearance window may be part of what makes a hangover drag on for so long.
Glutathione and Acetaldehyde: A Randomized Trial
Acetaldehyde — the direct product of ethanol oxidation — is itself toxic and a major contributor to hangover symptoms. Song et al. (2024) tested whether a 50 mg glutathione supplement could reduce those symptoms, running a randomized, double-blind, placebo-controlled crossover trial. Forty healthy participants (ages 19–40) drank whiskey to reach an alcohol intake of 0.78 g/kg body weight, with blood alcohol and acetaldehyde measured at seven time points over the following 15 hours.
The glutathione group showed lower acetaldehyde levels (p < 0.001). Interestingly, the hangover questionnaire did not show a statistically significant difference between groups — but the researchers considered the questionnaire too subjective to be a reliable measure, and viewed the acetaldehyde reduction as the more meaningful finding given acetaldehyde’s known role in hangover symptoms.
They also cited studies pointing to an alternate acetaldehyde-clearing pathway that becomes more relevant during chronic alcohol use. Acetaldehyde is normally oxidized to acetate by aldehyde dehydrogenase, but a CYP450 enzyme — CYP2E1 — can also do this job. The catch is that the CYP2E1 route generates more reactive oxygen species, which depletes the liver’s glutathione reserves as it works to neutralize them. Notably, some of the cited research found that glutathione doesn’t just get consumed as a side effect of this process, it can also directly metabolize acetaldehyde.
A Detour Into CYP450 Structure
That mention of CYP2E1 sent me down a bit of a rabbit hole into CYP450 enzyme structure — specifically, the different binding sites where the actual reactions take place, and the enzyme that works in partnership with them: NADPH-cytochrome P450 reductase.
Tomková et al. (2015), in their study on genetic variation in NADPH-cytochrome P450 reductase in a Czech cohort, cite multiple studies describing what’s known about its structure. The enzyme sits on the outer surface of the endoplasmic reticulum, and is active in the cytosol, with a hinge structure that draws NADPH, FAD, and FMN close enough together for electrons to pass between them in sequence.
What’s interesting is how selectively genetic variation can affect this system: a given polymorphism might impair the reductase’s ability to catalyze reactions with one specific CYP450 enzyme and substrate, while leaving its interactions with other CYPs completely unaffected. That’s a plausible piece of the puzzle for why some people metabolize certain drugs poorly while processing others entirely normally.
The Takeaway
Alcohol dehydrogenase’s promiscuity turns out to explain a lot — from the toxicology of industrial alcohol poisoning, to the slow burn of a hangover. And genetic polymorphisms in NADPH-cytochrome P450 reductase can determine how an individual’s body handles a given drug. It’s a good reminder that a single enzyme rarely tells the whole story; the surrounding cast of cofactors, competing substrates, and genetic variation that determines the outcome.
References
Jones, A. W. (2024). Elevated blood-ethanol concentration promotes reduction of aliphatic ketones (acetone and ethyl methyl ketone) to secondary alcohols along with slower oxidation to aliphatic diols. Archives of Toxicology, 1–7.
Song, G., Han, H., Park, S., Sa, S., Chung, W., & Lee, B. Y. (2024). Effects of GSH on alcohol metabolism and hangover improvement in humans: A randomized double-blind placebo-controlled crossover clinical trial. Nutrients, 16(19).
Tomková, M., Panda, S. P., Šeda, O., Baxová, A., Hůlková, M., Siler Masters, B. S., & Martásek, P. (2015). Genetic variations in NADPH-CYP450 oxidoreductase in a Czech Slavic cohort. Pharmacogenomics, 16(3), 205–215.
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