The Spark of Life: Electricity in the Human Body by Frances Ashcroft Page A
Authors: Frances Ashcroft
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recovering from what we learn is a near fatal dose of tetrodotoxin – ‘terrible stuff and very quick’. He survives only because his companion administered artificial respiration until medical help arrived. Bond also has an encounter with a blue-ringed octopus, whose bite is laced with tetrodotoxin, in the film Octopussy . As ever, 007 emerges from these incidents shaken but not stirred.
Red Tides and Suicide Potions
When conditions are right, spectacular blooms of the algae Alexandrium can occur that turn the sea the colour of blood. The deleterious effects of such red tides have been known for centuries and the biblical account of one of the great plagues of Egypt paints a vivid picture: ‘all the waters that were in the river were turned to blood. And the fish that was in the river died; and the river stank and the Egyptians could not drink of the water of the river.’ Such red tides are composed of millions of minute algae, known as dinoflagellates, which produce a number of virulent neurotoxins, including saxitoxin. Like tetrodotoxin, saxitoxin blocks sodium channels. Filter-feeding molluscs like mussels and clams may ingest the dinoflagellates, thereby concentrating the toxins they produce, and creatures that in turn feed on them may be poisoned. As much as 20,000 micrograms of saxitoxin per 100 grams of tissue (250 times the legally allowed limit) has been recorded in Alaskan mussels: at this level, consumption of a single mussel can kill you. Even more frighteningly, a single green shawl crab from the Great Barrier Reef can contain enough toxin to kill 3,000 people. Dinoflagellates are most abundant in spring and summer, due to the higher sunlight levels and warmer waters, which may be the origin of the old adage ‘Do not eat shellfish unless there is an R in the month’.
In developed countries shellfish poisoning is very rare, due to intensive surveillance programmes and stringent regulations which ensure that, once detected, the affected areas are quarantined and shellfish sales prohibited. In the last decade, seasonal outbreaks of paralytic shellfish poisoning (mainly due to saxitoxins) have led to temporary bans on the sale of shellfish from waters around the world. The Alaskan shellfish industry has been radically affected as the butter clam is toxic for large parts of the year. But while commercial seafood is safe, this is not necessarily the case for shellfish that people collect themselves. Between 1973 and 1992 there were 117 cases of paralytic shellfish poisoning in Alaska, 75 per cent of them between May and July. Fortunately only one person died, but many required hospitalization. The most dramatic outbreak in recent years happened in 1987 in Guatemala, when 187 people were affected by eating clams and 26 of them died.
Tetrodotoxin and saxitoxin are molecular mimics. Each physically plugs the external mouth of the sodium channel pore, is almost equally potent at inhibiting channel function, and produces similar physiological responses. Both are also valuable research tools because they block sodium channels rather specifically, leaving most other channels untouched. Tetrodotoxin is routinely used in scientific studies today to block sodium channels and enable other channels to be studied in isolation. Its action was discovered by Toshio Narahashi in 1962, working round the clock throughout Christmas and New Year with John Moore and William Scott. Narahashi recalls that the reviewer of their manuscript jotted down a request for some of the toxin at the bottom of his report. It was to be the first of many such requests.
By now you might be wondering why butter clams are not affected by the saxitoxin they contain and why puffer fish swim happily around, despite high tetrodotoxin levels. The answer is that the affinity of their own sodium channels for the toxin is dramatically reduced because evolution has changed one or more of the amino acids in the toxin-binding site. A similar mutation is
Red Tides and Suicide Potions
When conditions are right, spectacular blooms of the algae Alexandrium can occur that turn the sea the colour of blood. The deleterious effects of such red tides have been known for centuries and the biblical account of one of the great plagues of Egypt paints a vivid picture: ‘all the waters that were in the river were turned to blood. And the fish that was in the river died; and the river stank and the Egyptians could not drink of the water of the river.’ Such red tides are composed of millions of minute algae, known as dinoflagellates, which produce a number of virulent neurotoxins, including saxitoxin. Like tetrodotoxin, saxitoxin blocks sodium channels. Filter-feeding molluscs like mussels and clams may ingest the dinoflagellates, thereby concentrating the toxins they produce, and creatures that in turn feed on them may be poisoned. As much as 20,000 micrograms of saxitoxin per 100 grams of tissue (250 times the legally allowed limit) has been recorded in Alaskan mussels: at this level, consumption of a single mussel can kill you. Even more frighteningly, a single green shawl crab from the Great Barrier Reef can contain enough toxin to kill 3,000 people. Dinoflagellates are most abundant in spring and summer, due to the higher sunlight levels and warmer waters, which may be the origin of the old adage ‘Do not eat shellfish unless there is an R in the month’.
In developed countries shellfish poisoning is very rare, due to intensive surveillance programmes and stringent regulations which ensure that, once detected, the affected areas are quarantined and shellfish sales prohibited. In the last decade, seasonal outbreaks of paralytic shellfish poisoning (mainly due to saxitoxins) have led to temporary bans on the sale of shellfish from waters around the world. The Alaskan shellfish industry has been radically affected as the butter clam is toxic for large parts of the year. But while commercial seafood is safe, this is not necessarily the case for shellfish that people collect themselves. Between 1973 and 1992 there were 117 cases of paralytic shellfish poisoning in Alaska, 75 per cent of them between May and July. Fortunately only one person died, but many required hospitalization. The most dramatic outbreak in recent years happened in 1987 in Guatemala, when 187 people were affected by eating clams and 26 of them died.
Tetrodotoxin and saxitoxin are molecular mimics. Each physically plugs the external mouth of the sodium channel pore, is almost equally potent at inhibiting channel function, and produces similar physiological responses. Both are also valuable research tools because they block sodium channels rather specifically, leaving most other channels untouched. Tetrodotoxin is routinely used in scientific studies today to block sodium channels and enable other channels to be studied in isolation. Its action was discovered by Toshio Narahashi in 1962, working round the clock throughout Christmas and New Year with John Moore and William Scott. Narahashi recalls that the reviewer of their manuscript jotted down a request for some of the toxin at the bottom of his report. It was to be the first of many such requests.
By now you might be wondering why butter clams are not affected by the saxitoxin they contain and why puffer fish swim happily around, despite high tetrodotoxin levels. The answer is that the affinity of their own sodium channels for the toxin is dramatically reduced because evolution has changed one or more of the amino acids in the toxin-binding site. A similar mutation is
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