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The need for more accurate and precise exposure estimates led to the use of a suite of biochemical, physiological and histological biomarkers to indicate exposure. Most of these biomarkers are considered to indicate exposure but only some have also been linked to an adverse biological effect.

The biomarker probably most relevant to bleached kraft mill effluents involves a family of enzymes called the P450 system. Some of the P450 family of enzymes are found in aquatic animals and they convert endogenous and exogenous hydrophobic substances to more polar moieties, thus making them more water soluble and usually facilitating elimination and excretion. One of the enzymes, P4501A, responds to some chlorinated aromatic compounds as well as some aromatic hydrocarbons. When a fish is exposed to these chemicals, P4501A is induced (i.e., is produced) in the liver and other organs. Therefore, elevated levels of P4501A in fish from an area indicates exposure of those fish to inducing chemicals (Stegeman et al., 1992). The question then becomes whether or not this exposure results in an adverse effect.

Enzyme induction could be detrimental in several ways such as metabolism to a more toxic substance (some organophosphorous pesticides). Induction could also increase metabolism of endogenous chemicals which are necessary for the health of the species (steroid hormones, Andersson, et al., 1988b; Larsson, et al., 1987).

In a recent discussion of biomarkers and their utility, a number of criteria are given to objectively evaluate the strengths and weaknesses of biomarkers (Huggett et al., 1992). P4501A or EROD (as an indicator of mixed function oxidase [MFO] activity) is induced by a number of chemicals including some PCB's, polynuclear aromatic hydrocarbons and polychlorinated dioxins and dibenzofurans. It has also been shown to be induced by bleached as well as unbleached mill effluents (Lindström-Seppä et al., 1992; Pesonen and Andersson, 1992; Van Der Kraak et al., 1992; McMaster et al., 1992). Therefore it could be used as a biomarker of exposure to mill effluent.

Induction of P45O1A should not be used to denote exposure to a specific chemical, e.g., chlorinated dioxins, without further information. It has been postulated that reduced reproductive steroid hormones due to induced P4501A has affected reproduction in finfish (Andersson, et al., 1988b; Larsson, et al., 1987). In mammals, cytochrome P4501A has minimal activity on steroids (Nelson and Strobel, 1987; Waxman, 1988) but its expression appears to be controlled by the hormone estradiol in fish, rather than vice versa (Elskus, 1992). No evidence is available to show any linkage between induced P4SO1A and reproductive effects from secondary treated effluent from mills using 100% chlorine dioxide.

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