Enzyme:CYP2E1

CYP2E1 (cytochrome P450 2E1) is a hepatic drug-metabolizing enzyme of the cytochrome P450 superfamily, encoded by the CYP2E1 gene on chromosome 10q26.3. It is unusual among the clinically important cytochromes P450 in two respects. First, its substrates are characteristically small molecules, including ethanol, acetaminophen, the halogenated volatile anesthetics, and a long list of low-molecular-weight industrial solvents and procarcinogens. Second, the dominant determinant of its activity is not genetic polymorphism but induction, and the inducers include physiological and behavioural states (chronic alcohol use, fasting, obesity, uncontrolled diabetes) as much as they include other medicines. The single most clinically consequential fact about CYP2E1 is that it is the metabolic bridge between chronic alcohol use and acetaminophen hepatotoxicity.

The history begins with the discovery that the liver can oxidize ethanol by a route separate from alcohol dehydrogenase. In 1968, Charles Lieber and Leonore DeCarli, working at the Bronx Veterans Affairs Hospital and Mount Sinai in New York, reported that hepatic microsomes oxidize ethanol, and, crucially, that this microsomal ethanol-oxidizing activity increases adaptively after a period of ethanol feeding. They named the pathway the microsomal ethanol-oxidizing system (MEOS). The enzyme principally responsible for it was purified over the following years and eventually named CYP2E1. The second strand of the history is the parallel discovery, by James Mitchell, David Jollow, William Potter, and James Gillette at the National Institutes of Health, of how acetaminophen damages the liver: a four-part 1973 series established that acetaminophen hepatotoxicity is mediated by a reactive metabolite, and that glutathione is the protective species that detoxifies it before it can bind to and injure hepatocytes. CYP2E1 is a principal enzyme generating that reactive metabolite, and the convergence of the two histories, the inducible ethanol-oxidizing enzyme and the glutathione-dependent acetaminophen story, is the clinical core of this page.

Tissue distribution

CYP2E1 is most abundant in the liver, where it carries out the clinically dominant share of its metabolism. It is also expressed in extrahepatic tissues, including the brain, where it has been of research interest in relation to oxidative stress, and the kidney. CYP2E1 is notable for generating reactive oxygen species as a byproduct of its catalytic cycle, more so than most other cytochromes P450, which is one of the reasons it features heavily in the literature on alcohol-related and oxidative liver injury.

Function and substrate spectrum

CYP2E1 catalyzes hydroxylation, oxidation, and dehalogenation reactions, with a substrate preference for small, often hydrophilic molecules. The standard probe substrate is chlorzoxazone, whose 6-hydroxylation is used to measure CYP2E1 activity. Beyond the medicines listed below, CYP2E1 metabolizes and in many cases bioactivates a large set of low-molecular-weight industrial chemicals and procarcinogens, including benzene, styrene, vinyl chloride, carbon tetrachloride, and the N-nitrosamines; that activity is central to occupational and environmental toxicology but falls outside the medicine-focused scope of this table.

The table below collects the clinically important CYP2E1 medicine substrates with each entry tagged by the contribution CYP2E1 makes to overall clearance (or, for acetaminophen, to the toxic bioactivation route): major (CYP2E1 is the predominant route), moderate (CYP2E1 contributes meaningfully but other routes carry comparable load), minor (CYP2E1 contributes but other pathways dominate), and partial (one of several substantial routes). The list is curated for clinical relevance and is not exhaustive; see Comprehensive substrate and interaction tables below for the authoritative maintained resources.

near-complete CYP2E1 medicine-substrate table (click to expand)

Substrate	Therapeutic class	CYP2E1 contribution	Clinical notes
Acetaminophen	Analgesic / antipyretic (paracetamol)	partial (toxic route)	The clinically critical CYP2E1 substrate. At therapeutic doses acetaminophen is cleared mostly by glucuronidation and sulfation; a minor oxidative route, substantially CYP2E1-mediated, generates the hepatotoxic metabolite NAPQI. CYP2E1 induction (chronic alcohol, fasting) increases NAPQI formation. See Clinical implications.
Caffeine	Methylxanthine stimulant	minor	A minor 8-hydroxylation route; the dominant caffeine route is CYP1A2.
Chlorzoxazone	Muscle relaxant	major	The standard probe substrate for CYP2E1 activity (6-hydroxylation).
Dapsone	Antibiotic / antimycobacterial	partial	Mixed CYP2E1 + CYP3A4; relevant to dapsone-induced methemoglobinemia.
Enflurane	Halogenated volatile anesthetic	major	Defluorinated by CYP2E1; fluoride release is the basis of historical nephrotoxicity concern.
Halothane	Halogenated volatile anesthetic	major	CYP2E1-mediated oxidation generates the reactive trifluoroacetyl intermediate implicated in halothane hepatitis.
Isoflurane	Halogenated volatile anesthetic	major	Defluorinated by CYP2E1, but to a much smaller degree than enflurane or methoxyflurane.
Isoniazid	Antimycobacterial (tuberculosis)	partial	Substrate AND inducer of CYP2E1; the CYP2E1-mediated route is implicated in isoniazid hepatotoxicity.
Methoxyflurane	Halogenated volatile anesthetic (largely abandoned)	major	Extensively defluorinated by CYP2E1; the resulting fluoride-mediated nephrotoxicity is the principal reason methoxyflurane was abandoned as a general anesthetic.
Sevoflurane	Halogenated volatile anesthetic	major	Defluorinated by CYP2E1, releasing inorganic fluoride; clinically tolerated but historically scrutinised for renal effects.
Theophylline	Methylxanthine bronchodilator	minor	A minor route; the dominant theophylline route is CYP1A2.

Phenotype categories

CYP2E1 does not have a clinically actionable poor-metabolizer / intermediate / normal / ultrarapid phenotype classification, and there is no CPIC dosing guideline for CYP2E1. This is the strongest version of a disclaimer that also applies to CYP1A2 and CYP3A4: the variability in CYP2E1 activity between individuals and within the same individual over time is overwhelmingly driven by induction state, not by genotype. A person's CYP2E1 activity depends on their recent alcohol intake, their nutritional state, their body weight, and their glycaemic control far more than on which CYP2E1 alleles they carry. Pre-prescription CYP2E1 genotyping has no established clinical role.

Major variants

Several CYP2E1 variants have been catalogued and extensively studied in association with alcohol-related disease, cancer susceptibility, and other conditions, but none has a clinically actionable, reproducible effect on medicine dosing:

• \*1A, the reference allele.
• \*5B (the c2 allele, defined by the RsaI and PstI restriction-site polymorphisms in the 5' flanking region, rs2031920 and rs3813867), the most-studied variant. It has been associated with altered transcriptional activity in some studies, but the functional and clinical effects have been modest and inconsistent across populations.
• \*6 (the DraI polymorphism, rs6413432), similarly studied and similarly inconsistent.
• \*7, a promoter-region variant reported to affect inducibility.

The contrast with the strongly actionable polymorphisms of CYP2D6, CYP2C19, and TPMT is instructive: CYP2E1 is a clinically important enzyme whose importance is almost entirely a matter of induction biology rather than inherited variation.

Inducers

Induction is the heart of CYP2E1 pharmacology, and the mechanism is unusual. While CYP2E1 transcription can be modestly upregulated, much of its induction operates post-translationally: substrate binding stabilizes the CYP2E1 protein against degradation, so that exposure to a substrate raises the standing quantity of enzyme. The clinically important inducers are largely not medicines:

• Chronic ethanol use is the canonical CYP2E1 inducer. Sustained alcohol intake substantially raises hepatic CYP2E1, the adaptive increase that Lieber and DeCarli first described in 1968.
• Fasting and starvation induce CYP2E1, mediated by the ketone bodies that accumulate during fasting.
• Obesity and non-alcoholic fatty liver disease are associated with raised CYP2E1.
• Uncontrolled diabetes with ketosis induces CYP2E1.
• Isoniazid is a medicine that both is a substrate of and induces CYP2E1.

Inhibitors

• Disulfiram, through its metabolite diethyldithiocarbamate, is an inhibitor of CYP2E1, and has been used as a probe inhibitor in research settings.
• Acute ethanol is a competitive inhibitor of CYP2E1. This is clinically important and frequently misunderstood: while chronic alcohol use induces CYP2E1, a single acute alcohol exposure transiently competes with other substrates at the active site. This is the reason the timing of alcohol and acetaminophen exposure, not merely their co-occurrence, governs the hepatotoxic risk (see below).

Clinical implications, summary

Acetaminophen hepatotoxicity and chronic alcohol use. This is the headline CYP2E1 clinical story, and it is the product of two mechanisms acting together. Acetaminophen at therapeutic doses is cleared mostly by safe conjugation pathways, with only a small fraction oxidized, substantially by CYP2E1, to the reactive metabolite NAPQI; NAPQI is then detoxified by conjugation with glutathione. In a person who uses alcohol chronically, CYP2E1 is induced, so a larger fraction of an acetaminophen dose is converted to NAPQI. If that person is also fasting or malnourished, a common situation in heavy alcohol use, hepatic glutathione stores are depleted, so less NAPQI is detoxified. The combination, more NAPQI generated and less of it neutralised, means that acetaminophen can produce serious hepatotoxicity at doses at or near the upper end of the conventional therapeutic range, well below the doses required to injure a healthy liver. This is the established clinical-pharmacology account of why chronic heavy alcohol use is a recognised risk factor for acetaminophen liver injury.

A subtlety in this story is the timing point introduced in the Inhibitors section. Because acute ethanol competitively inhibits CYP2E1, alcohol present in the body at the same moment as acetaminophen can transiently reduce NAPQI formation. The period of greatest hazard is therefore not simultaneous co-ingestion but acetaminophen taken once the alcohol has been cleared, when the chronically induced CYP2E1 is no longer competitively occupied and is free to generate NAPQI from the acetaminophen dose.

Halogenated anesthetics. CYP2E1 defluorinates the halogenated volatile anesthetics, releasing inorganic fluoride. With methoxyflurane this fluoride release reached nephrotoxic concentrations and was the principal reason the medicine was abandoned for general anesthesia; with sevoflurane the same chemistry occurs at lower and clinically tolerated levels. CYP2E1-mediated oxidation of halothane generates a reactive trifluoroacetylated intermediate that is implicated in the immune-mediated liver injury of halothane hepatitis.

No genotype-guided dosing. There is no CPIC guideline and no routine genotyping for CYP2E1. The clinically actionable variables are the patient's induction state, principally their alcohol use and nutritional status, not their CYP2E1 genotype.

Comprehensive substrate and interaction tables

The substrate and interaction tables on this page are curated for clinical relevance, not for completeness. Three authoritative external resources maintain comprehensive lists of CYP2E1 substrates, inhibitors, and inducers, and the wiki recommends them to any reader who needs an exhaustive look-up:

• Flockhart Cytochrome P450 Drug Interaction Table, maintained by the Department of Medicine at Indiana University School of Medicine. The most widely cited clinical-reference cytochrome P450 table; substrate-, inhibitor-, and inducer-tiered, updated regularly. Available at https://drug-interactions.medicine.iu.edu/.
• U.S. Food and Drug Administration Drug Development and Drug Interactions Table, the regulatory-grade list FDA uses for labeling and clinical-trial design decisions. Smaller than Flockhart but every entry is FDA-vetted. Available via the FDA Center for Drug Evaluation and Research clinical drug interaction page.
• PharmGKB, the pharmacogenomics knowledge base hosted at Stanford University; the CYP2E1 gene page indexes substrate-, inhibitor-, and inducer-relationships with their underlying primary literature. Available at https://www.pharmgkb.org/.

For a detailed account of CYP2E1 in the specific context of alcohol-related and oxidative liver injury, the Lu and Cederbaum 2008 review in Free Radical Biology and Medicine is a standard reference.

See also

• Enzyme:CYP1A2, Enzyme:CYP3A4, Enzyme:CYP2D6, Enzyme:CYP2C19, Enzyme:CYP2C9, Enzyme:CYP2B6, Enzyme:CYP2C8, Enzyme:UGT1A1 (other drug-metabolizing enzymes covered in the wiki)
• Acetaminophen (the clinically critical CYP2E1 substrate)
• Disulfiram (the canonical CYP2E1 inhibitor)
• Category:Drug-metabolizing enzymes