Alterations in the hepatic biotransformation of steroids as a potential mechanism for contaminant induced changes in plasma testosterone concentrations in juvenile alligators (Alligator mississippiensis)

Alterations in steroid homeostasis have been reported in alligators inhabiting contaminated lakes in Florida. In this dissertation I examined one pathway through which contaminants could perturb plasma sex steroids in alligators from these lakes. Plasma steroid concentrations are regulated by means of a complex network of pathways that include gonadal synthesis and hepatic metabolism, which can lead to excretion via the urine. Hepatic enzymes tend to have broad substrate specificities and are responsible for biotransforming both endogenous substrates (such as sex steroids) and xenobiotics. I therefore hypothesized that organochlorine pesticides are up-regulating hepatic biotransformation enzymes in alligators inhabiting contaminated sites, thus leading to the increased metabolism and clearance of sex steroids. I examined whether differences in hepatic enzymes exist among contaminated and reference sites and if these alterations provide a mechanism through which plasma sex steroid concentrations can be modulated in alligators.;I found that variation in testosterone biotransformation enzymes exists among lakes in Florida with alligators from Lake Woodruff (reference site) exhibiting sexually dimorphic patterns of total testosterone hydroxylase activity. This pattern is not present in alligators from Lakes Okeechobee and Apopka, or in the central Everglades (contaminated sites). In subsequent studies, I report that sexually dimorphic patterns of a specific testosterone hydroxylase enzyme, cytochrome P-4503A (CYP3A), can be permanently disrupted through developmental exposure to a toxin (ethanol) and/or reversibly modulated through exposure to an organochlorine compound (toxaphene) during later life stages. Contrary to what I had hypothesized, plasma testosterone concentrations do not decrease 24 hours after exposure to CYP3A inducers, although a positive relationship between CYP3A and testosterone was observed. This suggests that gonadal steroidogenesis could be compensating for increased testosterone biotransformation and clearance due to CYP3A induction. I also examined if relationships between CYP3A and the nuclear receptors AR, ER, GR, SXR and PR exist, which would suggest a functional relationship between these genes. Future studies should examine urinary steroid clearance in response to CYP3A inducers as well as the contributions of the hypothalamus, pituitary, and nuclear receptors in the mediation of the indirect and direct regulation of hepatic enzymes in alligators.