Lipid

Lipid normalization (expressing the toxicant concentration as a lipid value; e.g., mg PAH in tissue/g lipid) may be useful for comparing responses among species on a body-residue basis for hydrophobic toxicants; however, if the compound is not lipid soluble (e.g., logio Kow < 2), then normalization may have little utility. The lipid content is important for many hydrophobic compounds because it often determines the rate of elimination, which is important for determining the bioaccumulation factor and time to a steady-state tissue concentration. Also, the more lipid an organism contains, the more a contaminant will bioac-cumulate. Essentially all PAHs exhibit a log Kow>2, therefore this is an important parameter, especially for those species that exhibit weak or nonexistent biotransformation for these compounds. It is not known how important lipid content is in those species that metabolize PAHs. Because these species do not accumulate PAHs, the bioaccumulation factor is not a useful parameter and the amount accumulated has little to do with the whole-body lipid content. It is possible however that lipid is important for these species because as PAHs are taken up, there is likely some partitioning occurring within the body before they are metabolized. In general, lipid normalization may improve comparisons between species because storage lipid will likely reduce the amount of chemical that is circulating in plasma, which is related to the amount that will reach the site oftoxic action (e.g., a specific enzyme or cell membrane).

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