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Elucidating the mysteries of natural toxins contained in marine organisms
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Professor Manabu Asakawa specializes in “toxins.” Although many people tend to have a negative image of toxins as something dangerous, he argues that when a substance does harm to humans we call it a “toxin,” but when it does good to humans we call it “medicine.”
Prof. Asakawa says “My research subjects are physiologically active natural substances contained in marine organisms. Among other things, I conduct various studies into tetrodotoxins and shellfish toxins.” Interestingly, tetrodotoxins and shellfish toxins are contained not only in puffer fish and shellfish, but also in other marine organisms that are taxonomically far distant from these species, and these instances are wide-ranging.
His interest is focused on one theme―“Why marine organisms other than puffer fish and shellfish contain tetrodotoxins and shellfish toxins?” |
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He states that “By looking into toxins from various angles, we may be able to know all about toxins, and if we understand all about toxins, we may discover valuable components that will benefit society. This is the attitude that I take to pursue my research.”
Why toxins accumulate, what is a toxic component, and what is the origin of toxins? ―To resolve these questions, the professor says, he is exploring the mysteries of toxins contained in marine organisms as his lifework, by making use of instrumental analytical techniques and other means. In recent years, Prof. Asakawa has gathered samples from the Seto Inland Sea, which is his familiar research location, and has also visited the Southwest Islands (the long string of islands stretching between Kyushu and Taiwan), primarily Ishigaki Island in Okinawa Prefecture, for the purpose of sample collection.
He says that “I have been collecting samples in the reefs near Kabira Bay, a world-renowned place of natural beauty. We can walk on the reef after the tide goes out, and about two hours from that point until the tide comes in is critical for sampling (and we are instantly deeply tanned, due to the strong sun).”
In Japan, there are only a few laboratories that study the toxins possessed by marine organisms. Prof. Asakawa’s laboratory is one of these laboratories. |
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Starting to pursue this field fascinated by the research of toxins, which offers exceptional thrills and is full of wonders.
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Shellfish toxins include paralytic shellfish poisons, diarrhetic shellfish poisons, and amnesic shellfish poisons. Among them, paralytic shellfish poisons, which cause paralytic symptoms in various parts of the human body, posed serious problems in Hiroshima Bay in the past. Diarrhetic shellfish poisons are the culprit of problems that have occurred in the Tohoku and Hokkaido regions. Also, amnesic shellfish poisons have become a matter of concern in North America (the United States and Canada); if intoxicated by these poisons, one can suffer from a memory disorder as an aftereffect.
According to Prof. Asakawa, the trouble is that paralytic shellfish poisonings may occur when people eat not only shellfish, but also other organisms, for example, certain types of crabs.
“Among crab species in possession of paralytic shellfish poisons, Zosimus aeneus and Atergatis floridus are well known in Japan. Their shellfish poisons are deadly since they have the same effects as tetrodotoxins. Therefore you had better not catch and eat crabs if you are not sure of their species, especially when you visit the Southwest Islands (which are also a habitat for gobies that contain tetrodotoxins),” warns the professor.
He cites a near-fatal accident as an example; there was a person who yearned to live on the Southwest Islands blessed with a bounty of nature, and thus moved from Japan’s mainland to Ishigaki Island. The person dived in the sea and caught a crab there, and ate miso soup with that crab, without knowing that it was toxic (Zosimus aeneus). The person nearly died from paralytic shellfish poisoning. While certainly Prof. Asakawa hopes that this research activities will help prevent such accidents, he admits that it is exactly “curiosity” that is the driving force behind his research.
“I feel an attraction to the substances which, even in a small amount, can show strong biological activity. Studying these substances excites me. I am very happy when I can extract an active substance in a pure form. I really enjoy doing my research.” says Prof. Asakawa with a carefree smile. |
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Initially he had studied microorganisms. However, after enrolling at the graduate school, where he started research on paralytic shellfish poisons, he recalls that he found himself absorbed in studying toxins in marine organisms.
He continues that “As for shellfish toxins, their origins can be explained using a pattern in which plankton produces a toxin, which is eaten by a bivalve, and then a human being eats the bivalve. In the case of crabs, however, we have completely no idea about the origins of their toxins. When walking on the reef while thinking about many things, sometime I happen to encounter strange phenomena.”
He expects that such a chance encounter may provide a clue to resolving certain problems and lead to the discovery of a “treasure” that will benefit society. To illustrate this, he introduces a case of a marine algae species living in the reef.
“After close observation, I found that a lot of two-to-three millimeter-long insects swarming around the marine algae. I brought some back to the university and had Professor Koichiro Kawai (Laboratory of Benthos Ecology, School of Applied Biological Science) investigate the insects. As a result, they were proved to be a kind of midges. I then thought that this phenomenon could be utilized for insect pest control, and undertook a research for this purpose. |
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Advancing overseas to conduct research on toxins—his great enthusiasm is evident in the attitudes willing to do low-profile work
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Furthermore, as part of his research regarding tetrodotoxins, Prof. Asakawa has recently been engaged in another task. That is the study of nemerteans, commonly known as “ribbon worms.”
He explains that “A ribbon-like worm that weighs only 0.5 gram or so (called “Cephalothrix simula”) contains a tetrodotoxin, whose quantity is staggeringly high. This is a dangerous worm that has a toxicity much higher than that in the ovaries and livers of puffer fish.”
The reason why such a large amount of tetrodotoxin is contained in this nemertean has not yet been discovered, which also arouses the interest of Prof. Asakawa.
He states that “To produce a tetrodotoxin crystal from the ovaries or livers of puffer fish, we need to collect a substantial amount of puffer fish internal organs and extract the crystal from these guts. Using this ribbon worm, however, we can extract a great amount of tetrodotoxin crystals from a test tube’s worth of the worm. This makes the extraction and purification processes easier.” With a twinkle of excitement in his eyes, he continues that “I strongly hope to clarify the mechanism for tetrodotoxin accumulation of this life form, whose weight is less than one gram.”
He has also expanded his research field overseas, conducing joint research on toxic crabs with a university in the Philippines. For this collaboration project, he has visited the Philippines for sampling.
The professor mentions that “I have primarily been engaged in studies to identify the causes of food poisoning due to toxic crabs that has occurred in the Visayan Islands (the Cebu, the Panay, the Negros, etc.) of the central Philippines. In some communities far away from urban areas, this type of food poisoning has occurred intermittently and at unpredictable times, when people consume a crab that they have caught, without knowing that it is a toxic one.”
After comparing poisonous constituents contained in toxic crabs living in the Philippines with those on the Ishigaki Island, he realized that the research has become even more complicated. Partly for this reason, it is extremely difficult to identify the origin of the toxin, according to the professor.
Nurturing younger researchers is also an important task. “The presence of natural toxics and other physiologically active natural substances are directly related to people’s lives, both in a positive and negative way. I therefore want students to persistently conduct this research. However, extraction and purification of a physiologically active substance entails time-consuming and low-profile work. I am afraid that students nowadays tend to prefer the kind of work that will produce immediately results…” says the professor, expressing the considerable difficulty of this task.
Prof. Asakawa expresses his hope saying that “I hope to share with many more students the sense of excitement and fulfillment that I myself enjoy.” His comment reflects a boundless enthusiasm, which is typical of a researcher. |
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| Manabu Asakawa |
Professor
Laboratory of Marine Bioresource Chemistry
April 1, 2002 – March 31, 2015 Associate Professor, School of Applied Biological Science, Hiroshima University
April 1, 2015 – present Professor, School of Applied Biological Science, Hiroshima University
Posted on Oct 13, 2015
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