Alteration of [¹⁴C]-Testosterone Metabolism After Chronic Exposure of Daphnia magna to Tributyltin

Abstract.

Tributyltin (TBT) is a marine biocide that has been shown to alter the activity of cytochrome P450 monooxygenases and elicit toxicity indicative of androgenization in some species. The present study was conducted to determine whether TBT altered P450-, reductase-, and transferase-mediated testosterone metabolic processes in Daphnia magna at sublethal exposure concentrations. Two generations of daphnids were continuously exposed for 21 days to nominal TBT concentrations ranging from 0.31 to 2.5 μg/L TBT. The highest TBT concentration (2.5 μg/L) was lethal to 60% of the exposed organisms. Lower TBT concentrations elicited no adverse effects on molting or reproduction of the daphnids. No differences were observed in the response of the first- and second-generation daphnids to the toxicity of TBT. The ability of daphnids to metabolize [¹⁴C]-testosterone in vivo was assessed following exposure of each generation to TBT. Production of hydroxylated, reduced/dehydrogenated, and glucose-conjugated metabolites of testosterone were all elevated following exposure of both generations to 1.25 μg/L TBT. These findings indicate that, under these conditions, TBT elicits no discernible effects on molting and reproduction of daphnids at sublethal concentrations, and testosterone metabolism is enhanced at concentrations approaching those that are lethal to organisms. Alterations of steroid metabolism by xenobiotics can be used as a more sensitive indicator of sublethal exposure in daphnids than reproductive endpoints.