Despite the evidence presented in the last slide, not all metabolites of endocrine disruptors are biologically more available or active. One of the arguments offered against concerns about potential human effects of bisphenol A (BPA) is that, at least in rats and mice, upon ingestion much of the parent BPA is quickly converted to BPA-monoglucuronide, which is a variant of BPA and has been shown to be biologically inactive (Matthews et al., 2001).

Furthermore, not all endocrine disruptors need to be altered biochemically in order to be biologically more available or active. A field study (Fairchild et al., 1999) indicated that nonylphenol can interfere with the normal growth and reproductivity of salmon smolts by mimicking the role of estrogen. The nonylphenol compound used in the study was an inert ingredient surfactant added to aid an insecticide to dissolve in water for easier spraying. The study showed that after exposure to nonylphenol, salmon smolts could not switch their osmoregulatory system from fresh water (where they hatch) to salt water (into which they migrate in the first fall of their life). Thus when the salmon smolts reached the ocean after migrating downstream, they died. This switch is normally mediated by estrogen.

The inability of the smolts to switch their osmoregulatory system was apparently disrupted by nonylphenol's estrogenic effects, as substantiated in a laboratory study by Madsen et al. (1997). Here it is the salmon’s osmoregulatory system that makes nonylphenol’s estrogenic effects more distinct and bioactive. (Note that a fish’s osmoregulatory system consists of a set of bodily structures and functions responsible for regulating the concentration of dissolved substances in its cells, despite changes of the concentrations in the surrounding medium.)