Metabolism of endogenous and exogenous anabolic agents in cattle.

The endogenous anabolic agents, estradiol-17 beta, progesterone and testosterone are steroids that are quickly metabolized by the liver and are not very active when administered orally. Estradiol-17 beta is excreted by the bovine in bile as free estradiol-17 alpha, and by swine, in urine, as glucuronides and sulfates. In ruminants, the primary metabolite of progesterone is pregnandiol and that of testosterone is epitestosterone. In horses, the metabolites of these compounds are primarily 17 ketosteroids. Esters of the endogenous anabolic agents are rapidly hydrolyzed and the nonesterified forms follow the same biotransformation pathways as the natural compounds biosynthesized by the animal. The exogenous anabolic agents, such as trenbolone acetate, zeranol and diethylstilbestrol, may be active by the oral route and are less readily metabolized in the liver than the endogenous anabolics. Their metabolic pathways may be complex and lead to excreted forms after glucuronide conjugation. With respect to biochemical mechanism of action, it can be assumed that the anabolics act like all steroids by way of intracellular receptors. Biotransformations could lead to more reactive molecules that may bind themselves to normal constituents of the organism. Bound metabolites are generally formed later than free metabolites, and are considered less toxic with a lower level of bioavailability.