Why are chickens so sensitive to dioxins, but terns seem much more resistant, despite their exposure through eating dioxin-tainted fish? The life-or-death difference researchers have found can be partially explained by two amino acids in the chain of 858 amino acids that form one critical protein.
The slight difference apparently changes the three-dimensional shape of the protein, known as the aryl hydrocarbon receptor, allowing dioxin to bind more easily to the chicken receptor’s dioxin-binding site, like a key in a lock, and trigger harmful effects. The dioxin “key” does not fit as smoothly into the tern receptor’s binding site.
The findings, reported recently in the Proceedings of the National Academy of Sciences, advance the possibility of a test that wildlife managers could use to assess dioxin sensitivity in wild animal populations, said Sibel Karchner, a researcher at Woods Hole Oceanographic Institution (WHOI) and lead author of the PNAS paper.
Dioxins are a group of chemical compounds with similar chemical structures and biological characteristics. Present in the environment worldwide, they are formed as an unintentional by-product of many industrial processes involving chlorine such as waste incineration, chemical and pesticide manufacturing, the production of PVC plastics and paper, and from forest fires and backyard burning. Dioxins and structurally related chemicals can cause an array of disorders in most vertebrate animals and have been linked to cancer and reproductive abnormalities in humans. Dioxin was the primary toxic component of the defoliant Agent Orange, and was found at Love Canal in Niagara Falls, NY.
In virtually all vertebrates, dioxin sticks or binds to the aryl hydrocarbon receptor, or AHR. Like any protein, AHR is coded for by a gene, whose DNA sequence dictates the exact sequence of amino acid “building blocks” joined together to make the protein. The particular sequence of amino acids determines how the protein folds into a specific three-dimensional shape.
Somewhere in that folded shape is a region where the dioxin molecule fits—the binding site. Once bound, the AHR-dioxin combination alters the function of other genes in the cell, triggering harmful effects. Without that “lock” region, the dioxin “key” cannot bind and trigger those harmful effects. Laboratory mice engineered to lack AHR are not affected by dioxin, while mice with AHR are poisoned by it.
Chicken and tern AHR proteins are 858 and 859 amino acids long, respectively. Karchner and colleagues used molecular cloning techniques to determine the entire sequence of amino acids for each receptor protein. They showed that the chicken AHR has a 7-fold higher affinity for dioxin than the tern AHR, providing a molecular explanation for at least part of the difference in sensitivity between these species. They then traced the difference in affinity to the dioxin-binding regions of the chicken and tern proteins. Additional experiments showed that only two of the 168 amino acids in this region were responsible for the difference.
“These two amino acids account for the entire difference in binding affinity for dioxin, and therefore have a major impact on the difference in sensitivity between chicken and tern,” Karchner said.
The AHR protein has been well studied in mammals, but has not been as extensively characterized in non-mammalian vertebrates. The protein has a similar structure in different animals, which suggests that it originated long ago in evolutionary time. “It is a very old protein, and many of the amino acid sequences in this region of the protein have been kept similar over timeβŽ―what is known as highly conserved sequences,” said Karchner. This high similarity allows the researchers to compare the AHR among different species. Cloning, in vitro expression, and analysis of protein function provide a promising method to study the potential impact of environmental contaminants on protected species.
“We looked at only two birds, the domestic chicken (Gallus gallus) and the common tern (Sterna hirundo),” Karchner said, “but you might be able to predict sensitivity of other wild animals by looking at a short section of a gene or protein sequence rather than conducting extensive studies on entire, large genes.” The authors report that other "less-sensitive" bird species share the same two critical amino acids as the common tern.
The research team included Karchner, Diana G. Franks and Mark E. Hahn of WHOI and Sean W. Kennedy from Environment Canada, National Wildlife Research Centre. In collaboration with Hahn and Karchner, Kennedy is continuing to collect samples from many species of wild birds to compare their AHRs. Hahn and others are pursuing similar studies in several species of marine mammals.
The study was supported with funding from the Woods Hole Sea Grant program.
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