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Abstract
1. The metabolism of trenbolone acetate, 17β-acetoxyestra-4,9,11-trien-3-one (TBA), an anabolic compound used as a growth promoter, was compared in rat and cow.
2. [6,7-3H] TBA was injected i.v. into rats and a heifer, and bile was collected for 24 h. In both species, the bile was the major route of excretion. TBA undergoes an extensive hydrolysis to 17β-hydroxyestra-4,9,11-trien-3-one and the unchanged compound was not detected, but subsequent major metabolic pathways are different in the two species.
3. In the rat, oxidation of the 17β-hydroxyl to the 17-oxo group and hydroxylation in the 16α-position are the major routes. The three major metabolites are 17β-hydroxyestra-4,9,11-trien-3-one, 16α,17β-dihydroxyestra-4,9,11-trien-3-one and 16α-hydroxyestra-4,9,11-trien-3,17-dione.
4. In the heifer, 17α-epimerization is the major pathway and the main metabolite is the 17α-hydroxyestra-4,9,11-trien-3-one.
5. In both species, estra-4,9,11-trien-3,17-dione and the other metabolites, resulting either from hydroxylation in 1, 2, 6β, 16α or 16β positions, or from aromatization of the A ring, were minor products.
6. Overall, 60% of the 3-oxotriene structures identified in the rat bile were 17β-hydroxylated and the remainder were 17-keto metabolites, whereas in the heifer bile 90% were 17α-hydroxylated compounds.
7. Thus, in bovine species, the major pathway is similar to those of testosterone or 17β-estradiol which are mainly excreted as their 17α-epimers. This epimerization strongly decreases the biological potency, as with the natural 17β-hormones, and leads to detoxication of tissue residues.