Environmental fate of endocrine disrupting growth promoters used in cattle production

Trenbolone acetate (TBA), melengestrol acetate (MGA) and zeranol are synthetic growth promoters extensively used in animal husbandry, yet despite occurrence in water and soil little is known about their environmental fate. The work provides some of the first details related to the persistence and mobility of these synthetic growth promoters and their metabolites (SGPM) in natural aquatic systems. In sunlit surface waters, results suggest that the families of TBA (including 17βtrenbolone, 17α-trenbolone and trendione) and MGA (also melengestrol) will readily undergo direct photolysis with half-lives between ~0.25-1 h in both natural and simulated sunlight. In contrast, zeranol, β-zearalanol and zearalanone only degrade via indirect photolysis pathways mediated by dissolved organic matter. Their transformation is attributable primarily to hydroxyl radical and triplet DOM at neutral pH values, whereas the contribution of singlet oxygen becomes greater in more alkaline waters. An observed pH-dependence for rates of indirect photolysis suggests these photooxidants react primarily with the monodeprotonated form of zeranol, as well as its metabolites. Given their high rates of direct phototransformation, close attention was paid to elucidating the photoproducts for the TBA family. Generally, all metabolites of TBA photodegrade primarily to a species hydroxylated at the C12 position of the steroid ring. While this 12-hydroxy trenbolone species is photostable, it can hydrolyzes to diand trihydroxy products. Most notably, however, it is also prone to dehydration, which results in the regeneration of the TBA metabolite from which it was derived. This dehydration pathway can be acidand base-catalyzed, but it also occurs to an appreciable extent at neutral pH. It is also temperature dependent, with greatest rates of parent metabolite regeneration at higher temperature. Consequently, the rapid rate of photolysis for TBA metabolites observed in daylight is followed by photoproduct-to-parent reversion mechanism in the dark indicating they will be far more persistent than currently realized.