| Synthetic growth promoters and their metabolites (SGPMs) can be released into the aquatic environment from agricultural crop, rangeland, and concentrated animal feeding operation (CAFO) runoff at concentrations higher than 1,000 ng/L, potentially impacting ecosystem health at concentrations as low as 10 ng/L. The available research on the environmental fate of these compounds and their transformation mechanisms is currently incomplete. To investigate aquatic degradation rates of the trenbolone acetate (TBA) metabolites 17alpha-trenbolone (17alpha-TBOH), 17beta-trenbolone (17beta-TBOH), and trendione (TBO), GC/MS/MS analysis was used to measure steroid loss as a function of time in biologically active microcosms. The effect of temperature and inocula was assessed at incubation temperatures of 5, 20, and 35°C using inocula collected across multiple seasons and water sources. High resolution LC/MS/MS was used to characterize transformation products and degradation pathways. Results of thorough kinetic analyses indicate that aerobic biodegradation yields half-lives of approximately 1.8, 0.68, and 1.3 days for 17alpha-TBOH, 17beta-TBOH, and TBO, respectively at 20°C. Results also indicate that seasonal changes in microbial activity can change these observed half-lives by as much as a factor of two, while changes in inocula source can drastically effect transformation observations. Incubation of 17alpha-TBOH at 5°C yielded a longer half-life (7.3 days) than samples incubated at 20°C, indicating that temperatures of 20°C and lower may have a detrimental effect on the enzyme-mediated transformations of TBA metabolites. Metabolite interconversion analysis indicates that up to 53% of TBO mass may transform into 17beta-TBOH, whereas only up to 7% of TBO mass may transform into 17alpha-TBOH. This stereo-specific pathway mimics patterns observed for analogous compounds in the estradiol and testosterone families of environmental steroids. Preference for 17beta-TBOH and TBO biotransformation over 17alpha-TBOH biotransformation could lead to preferential 17alpha-TBOH accumulation in affected water bodies. Although the results are expected to be system-specific, outcomes of this study will improve environmental risk assessment concerning the impact of synthetic growth promoters and their metabolites on aquatic organisms by improved characterization of the range of environmental transformations. |
