Chemical metabolism and toxicity to early life stages of fish

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Alkyl polycyclic aromatic hydrocarbons (PAH with one or more alkyl side chains) are found in crude oil, creosote, and contaminated sediments of rivers and harbours. Alkyl PAH are classified according to their parent structures and to the numbers of carbons in side chains. There are more than a dozen classes, and a single class, such as alkyl phenanthrenes, may include more than 500 unique compounds with 1 to 4 carbons in side chains. While little studied, they comprise more than 90% of total PAH in crude oil, and contributed to the impacts on fisheries of the Exxon Valdez oil spill. They are rapidly taken up by fish, which respond by producing cytochrome-P450 (CYP1A) enzymes. These enzymes add oxygen to alkyl PAH as the first step in their excretion. However, oxygenation may create metabolites that are more harmful than the parent compound. Hence, faster excretion carries a risk of greater toxicity, which is expressed as circulatory collapse and death of embryos and larvae, the most sensitive life stages.