Kinetic characterization of the inhibition of acyl coenzyme A:Steroid acyltransferases by tributyltin in the eastern mud snail (Ilyanassa obsoleta)

Exposure to tributyltin (TBT) has been causally associated with the global occurrence of a pseudohermaphroditic condition called imposex in neogastropod species. TBT elevates free testosterone levels in these organisms, and this upsurge in testosterone may be involved in the development of imposex. We investigated the ability of TBT to inhibit acyl coenzyme A:testosterone acyltransferase (ATAT) activity as well as microsomal acyl-coenzyme A:17β-estradiol acyltransferase (AEAT) in a neogastropod, the eastern mud snail Ilyanassa obsoleta as a mechanism by which TBT elevates free testosterone. TBT significantly inhibited both ATAT and AEAT activities in vitro at toxicologically relevant in vivo concentrations. Kinetic analyses revealed that TBT is a competitive inhibitor of ATAT (Ki = ∼9 μM) and is a weaker, noncompetitive inhibitor of AEAT (Ki = ∼31 μM). ATAT and AEAT activities associated with different microsome preparations were significantly correlated, and 17β-estradiol competitively inhibited the fatty acid esterification of testosterone suggesting that one enzyme is responsible for biotransforming both testosterone and 17β-estradiol to their corresponding fatty acid esters. Overall, the results of this study supply the much-needed mechanistic support for the hypothesis that TBT elevates free testosterone in neogastropods by inhibiting their major regulatory process for maintaining free testosterone homeostasis-the fatty acid esterification of testosterone.