Identification of dioxin-specific binding proteins in marine bivalves

Epidemiological investigations of germ cell tumors of Maine soft shell clams (Mya arenaria) and hard shell clams (Mercenaria spp.) from Florida demonstrate the prevalence of histogenically similar gonadal cancers as high as 40 and 60%, respectively. Human mortality rates due to ovarian cancer from the same areas are significantly greater than the national average. Since further investigation revealed that there was a heavy use of herbicides at these sites, we proposed to investigate their possible role in the etiology of the clam tumors. One avenue of investigation was to determine whether clams had a mechanism similar to the vertebrate Ah receptor which could be activated by halogenated aromatic hydrocarbons (HAH). We used the TCDD photoaffinity analog [125-I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin to detect two cytosolic proteins (28 and 39 kDa) in Mercenaria mercenaria and one (35 kDa) in Mya arenaria which specifically bound this ligand. Expression of both proteins in Mercenaria is tissue specific with the highest levels observed in cytosols from gill and gonad. Females exhibit higher levels of the 39 kDa protein in gonadal tissue than do the males. Studies to determine the relationship of these proteins to vertebrate Ah receptors, as well as their possible role in gonadal tumor development, are in progress.