LEBLANC, G.A.; STERNBERG, R.; GOODING, M.P.; GUAL, G.J.; North Carolina State Univ.: Testosterone-Fatty Acid Esterification: A Unique Target for the Endocrine Toxicity of Tributyltin to Gastropods

Contamination of the marine environment with tributyltin (TBT) has caused a global occurrence of endocrine toxicity – imposex – whereby female gastropods express male sex traits. Imposex has been causally associated with increased testosterone levels. We have previously demonstrated that free testosterone levels are regulated in the mud snail (Ilyanassa obsoleta) by esterification of excess hormone to fatty acid. We also have demonstrated that exposure to TBT causes a decrease in testosterone-fatty acid ester levels with a commensurate increase in free testosterone. In the present study, we demonstrate that acyl coenzyme A:testosterone acyltransferase (ATAT), the enzyme responsible for the esterification of testosterone, functions as a low affinity/high capacity enzyme to regulate free testosterone levels over a wide range of testosterone concentrations. ATAT activity is coordinated with the reproductive cycle of the snails causing seasonal differences in free testosterone levels between sexes. TBT does not directly inhibit ATAT activity at concentrations shown to elevate testosterone and cause imposex. Experiments are underway to define the precise mechanism by which TBT alters the ratio of free:esterified testosterone in snails. Testosterone is not currently known to function as a hormone in gastropods. Thus, experiments are being performed to establish whether mud snails possess an androgen receptor. Schatchard analyses has revealed the presence of high affinity (Kd=0.7 nM) testosterone binding activity. Future analyses will include screening the snail genome using PCR to identify and characterize androgen receptor-like gene sequences.