Toluene toxicity
Toluene is metabolized mainly to hippuric acid and can cause neurotoxicity.
Toluene toxicity encompasses the harmful effects that toluene can have on the body. This substance is notable for its widespread use as an industrial solvent and for the serious health risks associated with both occupational exposure and deliberate inhalation.
Quick Facts
- Exhaled unchanged
- 25%–40%
- Primary urinary metabolite
- hippuric acid
- Primary enzyme in hydroxylation
- CYP2E1 (44% of metabolism)
- Benzoyl glucuronide elimination
- 10–20% of benzoic acid elimination
Facts from the source article.
Metabolism in humans
A substantial portion of inhaled toluene, 25%–40%, is exhaled unchanged via the lungs, while the remainder is metabolized. The primary metabolic route involves hydroxylation to benzyl alcohol by cytochrome P450 enzymes, particularly CYP2E1, which accounts for roughly 44% of toluene metabolism. Other CYPs involved include CYP1A2, CYP2B6, CYP2C8, and CYP1A1; however, ethnic variability exists, as in the Finnish population where CYP2B6 is the primary enzyme. CYP2E1 catalyzes the formation of both benzyl alcohol and p-cresol, whereas CYP2B6 produces comparatively little p-cresol. Benzyl alcohol is believed to be converted to benzaldehyde by CYP rather than alcohol dehydrogenase, though this view is not universal. Benzaldehyde is then metabolized to benzoic acid, mainly by mitochondrial aldehyde dehydrogenase-2 (ALDH-2), with a small fraction handled by cytosolic ALDH-1. Benzoic acid is further metabolized to either benzoyl glucuronide (10–20% of elimination) or hippuric acid (benzoylglycine). Hippuric acid is produced via two steps: conversion of benzoic acid to benzoyl-CoA by benzoyl-CoA synthase, followed by conversion to hippuric acid by benzoyl-CoA:glycine N-acyltransferase. Ring hydroxylation to cresols is a minor pathway; most cresol is excreted unchanged in urine, though some p-cresol and o-cresol appear as conjugates. There is dispute over whether m-cresol is produced as a metabolite.
Measure of exposure
Hippuric acid has traditionally been used as a biomarker of toluene exposure, but its validity is questioned because endogenous production varies with diet, medical treatment, and alcohol consumption, making it insufficiently sensitive to distinguish exposed from non-exposed individuals. Urinary o-cresol is not detectable in unexposed subjects, offering greater specificity, but it is less sensitive and accounts for less than 1% of total toluene elimination, limiting its reliability. More recently, benzylmercapturic acid, a minor metabolite derived from benzaldehyde, has been suggested as the best marker: it is absent in non-exposed individuals, more sensitive than hippuric acid at low concentrations, unaffected by eating or drinking, can detect exposure down to approximately 15 ppm, and shows a better quantitative relationship with toluene exposure than hippuric acid or o-cresol.
Effects of long-term exposure
Chronic occupational exposure and toluene abuse from deliberate solvent inhalation are associated with serious adverse behavioral effects. Long-term exposure can lead to psychoorganic syndrome, visual evoked potential abnormality, toxic polyneuropathy, cerebellar, cognitive, and pyramidal dysfunctions, optic atrophy, hearing disorders, and brain lesions. Neurotoxic effects, particularly from repeated withdrawals, may cause postural tremors by downregulating GABA receptors in the cerebellar cortex; treatment with GABA agonists such as benzodiazepines provides some relief, and ventral intermediate nucleus thalamotomy is an alternative. These tremors appear irreversible and progressive, continuing after solvent abuse stops. Low-level exposure may disrupt astrocyte precursor cell differentiation, posing a risk to fetal neuronal development if pregnant women are exposed during critical stages.
More in Toxic Effects of Substances Chiefly Nonmedicinal As to Source
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced