WARD, Chelsea K. *; FONTES, Cris; MENDONCA, Mary T.; Auburn University; Auburn Univerisity; Auburn University: The role and characterization of corticosteroid-binding globulin in southern toads exposed to coal fly ash

Corticosteroid-binding globulin (CBG) is a protein that binds corticosterone (B) and is thought to render the steroid biologically inactive. During chronic stress, CBG may modulate the prolonged, and usually deleterious, elevation of B. However, the actual role of CBG during a stress response is not well understood. To determine if CBG changes may mitigate the effects of exposure to a chronic pollutant, we examined changes in CBG levels in southern toads, Bufo terrestris, when they were exposed to coal fly ash, a byproduct of coal burning power plants, over a 5 month period. Coal fly ash is known to contain numerous toxic trace metals and cause extended periods of elevated B. To conduct this study, we first characterized the properties of the CBG protein in southern toads. Toad CBG has a Kd of 25.2 nM, and binds B, testosterone and progesterone with high affinity. The rank order potencies for steroid inhibition of tritiated corticosterone are: B > progesterone > testosterone > dihydrotestosterone > estrogen > dexamethasone. After characterization, we monitored the changes in levels of CBG, total B, and free B in male toads that were exposed to either coal fly ash substrate and contaminated food or control substrate and non-contaminated food. Exposed toads experienced a significant peak in B 4 weeks after initial exposure to the toxicant. Free B also increased significantly at this time. CBG increased simultaneously with the total and free B peak, but not enough to nullify the significant increase in free corticosterone. This finding indicates that, in this species, CBG may be acting as a reservoir, but may not be ameliorating the effects of the B peak during the chronic stress response caused by long-term toxicant exposure.