Study of Marine NaturalProduct Chemistry
from Keio's Faculty of Science and Technology Unique and new compounds
derived from nature

Unknown substances from marine organisms are intriguing

How many substances are there on our earth? Associate Professor Kiyotake Suenaga of the Natural Product Chemistry lab makes an almost endless pursuit of unknown substances available from marine organisms, saying: “There must be an incredibly large number of effective substances yet to be discovered.” A recent example of his discoveries is a substance isolated from cyanobacteria. It has been found that this substance has an action that is expected to be used for remedies against cancer and osteoporosis, thus opening up the possibilities of application for pharmaceutical development.

A beach in Okinawa in May . . . A group of people, each wearing a longsleeved shirt and a hat, was found intently looking for something under the glaring sunlight. This group was Associate Professor Suenaga and his students collecting marine organisms. The quest of unknown substances begins with an incredibly exhaustive work of gathering potential materials (Fig. 1). 　Since Dr. Suenaga transferred to Keio University in 2006, he has been mainly collecting marine cyanobacteria. Cyanobacteria are a type of bacteria that have chlorophyll and perform photosynthesis. They are believed to have produced oxygen in the primeval times of the earth. Dr. Suenaga says that he chose cyanobacteria as his objective not because he felt special potential in cyanobacteria. “I had no particular theoretical reason for choosing cyanobacteria as my objective. No one knows that an exciting substance can be found in a particular organism,” remarks Dr. Suenaga. Indeed, assumption is a taboo when it comes to exploration of unknown substances. He continues: “If I dare to give a reason, it’s perhaps because cyanobacteria were attached to seaweeds which were the food of sea hares I was studying as a college student.” Sea hares are known to have a unique substance, which is considered to be their food origin. That’s why cyanobacteria have weighed on his mind since then.

The total amount of cyanobacteria that has been gathered in the past five years is approximately 150 kilograms, from which essence is extracted, allowing a “target” substance to be isolated. Isolation is a process of purifying (Fig. 2), making full use of chromatography＊1 and other sophisticated techniques in the state where no substances other than the target are mixed. The bench mark for Dr. Suenaga to choose a target is whether the substance in question has an anticancer activity ＊2 or not. Therefore, most of the substances Dr. Suenaga has chosen thus far have an anticancer activity.
Then he proceeds with the examination of the target substances based on their chemical structures. Use of a nuclear magnetic resonance spectrometer (NMR) installed at the Keio campus enables a substance’s chemical structure to be identified in a short period of time. If the chemical structure of a given substance is too complex, an NMR with higher resolution, available outside the campus, may be used. More specific aspects of the substances are determined based on their reactivity and crystal structure analysis. Once the structure has been identified, you can determine whether the substance is really unknown.
Bisebromoamide was the first substance discovered from an extract of cyanobacteria through such an exploration process. “We named it ‘Bisebromoamide’ because it was an amide containing bromine (Br) and found in cyanobacteria which were gathered in the seaside Bise district of Okinawa,” says Dr. Suenaga. He makes it a rule to give a substance he has discovered a name based on the place of discovery and/or the main characteristic of its molecule. Bisebromoamide was successively followed by the discovery of biselyngbyaside and leptolyngbyolides. And even a fourth substance is likely to be discovered before long.
That said, each of these substances is available in only an extremely small amount (several milligrams to several tens of milligrams). This makes it difficult to make detailed investigation into properties and actions of these substances. As a solution to obtain sufficient amount of substances, Dr. Suenaga is attempting artificial chemical synthesis. Synthesis methods for bisebromoamide and biselyngbyaside are expected to be established soon.

Dr. Suenaga’s research work now appears to be progressing smoothly. But he could not find any promising substance at all in the first two years. There are countless marine organisms. Why didn’t he switch to other organisms? “To tell the truth, I had been dealing with other marine organisms in my early years, but I couldn’t achieve visible results. That’s why I switched to cyanobacteria. Therefore, the idea of giving up cyanobacteria in a mere two years or so didn’t occur to me at all.” It seems that exploration of unknown substances also requires tenacity and toughness.
A turning point came to him when he began collecting cyanobacteria of different varieties from what he had been focusing on. “When talking about cyanobacteria, we tend to think, first and foremost, of those outstanding ones like impressive green algae. Having spent two years looking only at cyanobacteria, however, I suddenly noticed that cyanobacteria also exist that are plain and less conspicuous,” he remarks. This change in approach fueled his advance.

“Among the many substances I know, bisebromoamide is unique in that it contains D-amino acid, bromine (Br) and t-butyl group.” Substances derived from marine organisms are intriguing not merely because of their structures. Dr. Suenaga continues, “Recently I’m often asked by researchers of my acquaintance, ‘Do you have any interesting substance?’ So I send a sample. Later on, I receive a report from that researcher saying that he/she found a particular action.” For example, Prof. Je-Tae Woo of Chubu University informed him of the possibility that biselyngbyaside might become an effective remedy for osteoporosis.
Osteoporosis is a disease for which no effective remedy has been found to date. So biselyngbyaside has come to the fore as the key substance for an osteoporosis remedy. Dr. Suenaga humbly says, “Although I can gather and isolate unknown substances, I’m not in an environment where I can examine all of them in detail as to what actions these substances have. So I really appreciate other researchers’ efforts and support in this regard.” Despite his humble comment, it may not be an exaggeration to say that such a co-research framework can exist only because the substances discovered by Dr. Suenaga have been intriguing enough.
On March 23, 2011, the Chemical Daily ran an article reporting that fullscale pharmaceutical development endeavors would be undertaken using the substance discovered by Dr. Suenaga. Adding “Pharmaceutical development requires the highest possible safety and efficacy,” he appears to be not placing too much expectation on the proposed phar maceut ica l de velopment and is determined to steadily pursue his research work. Yet, at the same time, he proudly says “Should it fail to come to fruition as a new drug, the significance of continuing in-depth quest of unknown substances remains unchanged because it will eventually help to shed light on mysteries of life phenomena.”
“Many marine organisms have no shells to protect themselves and are slow in moving, making them apparently defenseless. Despite that, they are surviving. Decades ago, some scientists thought that such organisms must have a certain kind of defensive substance that enables them to survive despite their apparent weakness. This is believed to be the starting point of exploration of unknown substances peculiar to marine organisms. I’m not sure if this hypothesis is true or not, but as far as my research endeavors are concerned, I can confidently say that marine organisms do have many intriguing substances that are not yet known to us,” concluded Mr. Suenaga. Natural product chemistry is bound to attract more and more attention in the years to come.

I was born in Aizu in Fukushima Prefecture and raised in Sendai in Miyagi Prefecture. Since I grew up in a leisurely,stress-free environment, there was nobody who attended a cram school. Also, I was never told by my parents to study hard. So until I graduated from junior high school, I had rarely studied at home.
As a small boy, I thought to myself that I would find employment with the Japan National Railways (now JR) in the future. For me this seemed to be a natural course of life since three generations of my family, from my grand-grandfather to my immediate father, had worked for the National Railways. So I was raised in official National Railways housing. The fact is, my father was one of the first-generation drivers of the Tohoku Shinkansen SuperExpress. When I was an elementary school boy, my father was studying hard to become a Shinkansen driver to prepare for the Tohoku Shinkansen line opening, which I still remember. My own son appears to respect his grandfather much more than his father (myself) because he has no idea about what I’m doing.

I was once reaching for the universe. In fact, my interest had been in physics rather than chemistry until I became a university student. But by the time it was time for me to choose a lab to study in, my interest had shifted to natural products and other complex substances. At Nagoya University in those days, however, studying in the organic chemistry group of the Graduate School of Chemistry required a kind of resolution. It was because there were two professors’ labs available – Prof. Ryoji Noyori who was later to win a Nobel Prize, and Prof. Kiyoyuki Yamada, my teacher, both famous for being very strict. I finally decided to join Prof. Yamada’s lab.

Actually, I was already a research associate by that time. I quit the doctor’s course in February of the first year to fill the research associate’s post because the former research associate transferred to another lab as an associate professor. Whether willing or not and given little time to think about my future, I found myself hastily starting a researcher’s career.

You may call me a strict teacher. Every week we have one rinko session where lab students take turns reading a textbook written in English, a magazine meeting where students introduce scientific journals they have read, and a study meeting where they learn about instrumental analysis. Of the over ten lab students, five take charge of such meetings every week, meaning that one student will take charge of one meeting every two to three weeks. This is a rather heavy burden on the students as it requires significant amount of preparatory study.
Experiments are one important thing, but there is another more important thing; I would like my students to build up their academic capability by learning broadly. Concerning our specialty, I’d like them to acquire comprehensive knowledge of organic chemistry. Suppose a student who is engaged in isolation and structural determination lacks knowledge of organic synthesis reactions, or a student who is focused on synthesis of a natural product knows nothing about biosynthesis. I don’t want my students to be like that.
Looking back at myself, I was thoroughly educated during my student days. In those days, we were not allowed to cut corners in experiments for the sake of learning at the desk. So it was often the case that I studied at the desk while completing preparations on the testing bench so a reaction would take place. A research report meeting was scheduled on one Monday but my experiment conducted on Friday of the preceding week didn’t go well, which annoyed me. At the time, one of my seniors said, “Have you ever thought about why a research report meeting is held on Monday?” I understood his suggestion was that I should do the experiment over the weekend. I don’t say such a thing to my students, but our lab’s study meeting held on Saturday may be because of that experience during my student days. In retrospect, many of the tough experiences I had as a student are now proving to be assets for me. A friend of mine who found employment with a private company says the same thing.