Bullfrog Protein Protects Mice From Deadly Shellfish Toxin

A protein made by American bullfrogs helped protect mice from lethal doses of saxitoxin, the powerful poison responsible for paralytic shellfish poisoning, according to research published in Nature Communications on July 16. Saxitoxin is produced by harmful algal blooms and can build up in shellfish. Contaminated seafood usually looks, smells and tastes normal, and cooking or freezing does not destroy the toxin. Symptoms can begin about 30 minutes after eating it and include nausea, vomiting, numbness, high blood pressure and paralysis. Death can follow within hours. The illness is fatal in about 8.5 percent of cases, with roughly 2,000 people becoming sick worldwide each year. There is currently no known antidote. The toxin disrupts sodium ion channels on cell surfaces, interfering with nerve signals that control muscle movement. Researchers led by Daniel Minor, a biophysicist at the University of California, San Francisco, tested saxiphilin, a protein found in American bullfrogs and other amphibians. Scientists had previously suspected that the protein helps these animals resist saxitoxin by binding to it. In the experiments, 13 control mice received the toxin and nearly all developed symptoms, including limb paralysis, within three minutes before dying. By contrast, a single dose of saxiphilin given one minute after exposure allowed nine of 10 tested mice to survive the toxin’s lethal effects. The researchers also examined whether the protein could work when administered before or at the same time as poisoning. The team described saxiphilin as a kind of molecular sponge. After binding to saxitoxin, it appeared to carry the poison toward organs involved in removal from the body. The highest protein levels were found in the kidneys, followed by the heart and liver. Smaller amounts reached the brain and skeletal muscle, which may help explain the protection against paralysis. Rebecca Tarvin, an evolutionary biologist at the University of California, Berkeley, who was not involved in the study, said the researchers’ testing of prevention, toxin neutralization and post-exposure rescue provided a broad assessment of the protein’s potential. However, the result has been demonstrated in mice, not humans. Further research is needed to assess safety, determine the best way to deliver saxiphilin and establish whether it works against different paralytic shellfish toxins.