Mercacturic Acid figure 64-1 Pathways of acetaminophen metabolism.

to produce the active toxin: parathion is biotransformed to paraoxon, a stable metabolite that binds to and inactivates cholinesterase (see Chapter 8). Some drug metabolites are not chemically stable and are referred to as reactive intermediates; consider the metabolite of acetaminophen (Figure 64—1), which binds to nucleophiles such as glutathione; when cellular glutathione is depleted, the metabolite binds to cellular macromolecules, the mechanism by which acetaminophen kills liver cells (see Chapters 3 and 26). Both parathion and acetaminophen are more toxic under conditions in which CYPs are increased, such as following ethanol or phenobarbital exposure, because CYPs produce the toxic metabolites.

Some chemicals can be activated in the skin by ultraviolet (UV) and/or visible radiation. In photoallergy, radiation absorbed by a drug, such as a sulfonamide, results in its conversion to a more potent allergen than the parent compound. Phototoxic reactions to drugs, in contrast to pho-toallergic ones, do not have an immunological component. Drugs that are either absorbed locally into the skin or have reached the skin through the systemic circulation may be the object of photochemical reactions within the skin; this can lead directly either to chemically induced photo-sensitivity reactions or to enhancement of the usual effects of sunlight. Tetracyclines, sulfonamides, chlorpromazine, and nalidixic acid are examples of phototoxic chemicals; generally, they are innocuous to skin if not exposed to light.

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