Xenobiotic Metabolism

Xenobiotic metabolism is the set of enzymatic processes by which an organism transforms foreign compounds — drugs, plant toxins, pollutants — into forms it can excrete. It splits conventionally into phase I reactions, mostly oxidations by cytochrome P450 enzymes that expose a reactive group, and phase II conjugations, which attach glucuronic acid, sulfate or glutathione to make the product water-soluble. Phase I sometimes produces a metabolite more toxic than the parent compound, so the pathways matter individually.

Cats are the standard example of a species gap. Feline glucuronidation is deficient — the relevant UGT genes are pseudogenes — so compounds humans conjugate and excrete uneventfully accumulate instead. Paracetamol is the clearest case: without adequate glucuronidation more of the drug is diverted through an oxidative route to a reactive metabolite that exhausts glutathione, damages the liver, and oxidises haemoglobin to methaemoglobin. Doses ordinary for a person are lethal for a cat. Sensitivity to phenolic compounds and some essential oils follows from the same deficit.

The general principle is that "natural" and "safe in humans" are properties of a compound plus a metabolism, not of the compound alone. Theobromine clears slowly in dogs; birds are exquisitely sensitive to inhaled fumes; the direction of the difference is not predictable from relatedness, so extrapolation between species needs a Plausible Mechanism rather than analogy.

Everything here is dose and clearance: toxicity depends on amount relative to body mass and on whether intake outpaces elimination — arithmetic worth doing as Fermi Estimation before assuming a small exposure harmless. The same kinetics govern anaesthetic dose limits in Regional Nerve Block, the withdrawal timing behind Physical Dependence, systemic Retinoid safety, and the additive rules touched on in Retronasal Olfaction.