Low Dietary Uptake Efficiencies and Biotransformation Prevent Biomagnification of Octamethylcyclotetrasiloxane (D4) and Decamethylcyclopentasiloxane (D5) in Rainbow Trout

With octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane
(D5) being considered for evaluation under the UN Stockholm Convention
on Persistent Organic Pollutants, which specifically acknowledges
risks of biomagnification of persistent organic pollutants in traditional
foods, a study into the mechanism of the biomagnification process
of D4 and D5 in Rainbow trout was conducted by combining the absorption–distribution–metabolism–excretion
for bioaccumulation (ADME-B) approach to determine intestinal and
somatic biotransformation rates and radiochemical analyses to identify
metabolite formation. High rates of intestinal biotransformation of
D4 and D5 (i.e., 2.1 (0.70 SE) and 0.88 (0.67 SE) day–1, respectively) and metabolite formation [i.e., 52.0 (17 SD)% of
D4 and 56.5% (8.2 SD)% of D5 were metabolized] were observed that
caused low dietary uptake efficiencies of D4 and D5 in fish of 15.5
(2.9 SE)% and 21.0 (6.5 SE)% and biomagnification factors of 0.44
(0.08 SE) for D4 and 0.78 (0.24 SE) kg-lipid·kg-lipid–1 for D5. Bioaccumulation profiles indicated little effect of growth
dilution on the bioaccumulation of D4 and D5 in fish and were substantially
different from those of PCB153. The study highlights the importance
of intestinal biotransformation in negating biomagnification of substances
in organisms and explains differences between laboratory tests and
field observations of bioaccumulation of D4 and D5.