What is a Natural Product? by Amy E. Wright, PhD Director, Division of Biomedical Marine Research Harbor Branch Oceanographic Institution Every cell is composed of chemicals, and all of the chemicals in or associated with a given organism can be classified into two categories. The first category, primary metabolites, is chemicals that an organism absolutely has to have to survive. These include substances such as oily lipids, which make up cell membranes and enzymes, which are the proteins that carry out chemical reactions within cells. In contrast the second category, called secondary metabolites, is chemicals not absolutely essential for sustaining life, though they can preserve life. These are compounds with, typically, low molecular weight compared to primary metabolites. They are produced by a plant, microorganism or animal and presumably confer some type of evolutionary advantage in acting as defense mechanisms or serving other purposes. These compounds, which include things like caffeine and morphine, are commonly known as "natural products," and they are the focus of this and all of HARBOR BRANCH's drug discovery expeditions. Natural Products in the Marine Environment It takes a lot of energy to produce a natural product, so, why do the organisms bother? To better understand this process, we need to look at how animals grow and the habitats in which they live. On this expedition, we will be working primarily with invertebrates, such as soft corals and sponges. If you look at most reefs, you can see that almost every square inch of space is inhabited with such organisms making competition for space and food enormous. Most of the animals in question are sessile as adults, meaning they grow attached to something else, perhaps a rock or another animal. A few invertebrates, such as sea slugs, can move around, but they don't set any Olympic speed records, in fact the slowest fish are hundreds of times faster. As larvae, most of these invertebrates can get around, but once they enter their juvenile phase, they select a home, settle there, and metamorphose into adulthood if they have chosen a suitable spot and all goes well. In general, these animals can't get up and move if the neighborhood gets bad. This presents a number of problems: How can they defend themselves against things that want to eat them? How can they keep other animals from moving into their space? How can they find a mate if they can't go looking? How can they protect themselves from bacterial, fungal or viral infections? Secondary metabolites can be used to accomplish all of these tasks. If you taste bad, smell bad or are toxic, you are less likely to get eaten. Even if a fish has to take a bite out of a sponge to figure out that it is not something good to eat, the rest of the sponge can survive. Many fish eventually learn what is and is not tasty and stop even taking that first bite of animals with natural products that are effective deterrents. Secondary metabolites can be equally effective at keeping the neighbors from moving in on you. Most of these animals are filter feeders, meaning they take in water and remove whatever food is present, and so they compete with each other for the same food source, namely the water that surrounds them. It makes sense to keep the neighbors away so they won't filter out food before you do. Some of these animals, such as the sponge, even go so far as to produce toxic compounds that eventually kill off animals that try to set up house near them. Siphonodictyon coralliphagum is a sponge that exhibits this behavior. It bores through coral heads and produces a toxic substance that can kill the coral, allowing the sponge to colonize more space. Reproduction can also require some creative chemical assistance if you cannot move around and you live in a water environment. Many of the sessile invertebrates are broadcast spawners, meaning that at certain times of the year all the organisms in a given region of the same species simultaneously release sperm and eggs into the water column. Is it just chance that allows the sperm to find the eggs in the reproductive cloud formed? For at least one soft coral that lives in Australia, it has been demonstrated that a series of natural products present in the eggs actually act as sperm attractants. Natural products can also act as chemical "lighthouses" to guide larvae back to the adult organism that spawned them. The larvae can sense these compounds and actively swim back to the parent, thus insuring that they settle in a suitable habitat. On the other hand, these compounds may act to stop the larvae of other species from settling on top of the original inhabitants. A final potential role for secondary metabolites is in maintaining animals' health. Just like mammals, marine organisms can suffer from infections because bacteria, fungi and viruses are all present in marine habitats. This can be especially troublesome because most sessile invertebrates have primitive immune systems, if any. Many secondary metabolites are strong antibiotics, antifungal and/or antiviral agents, though, and this may be another reason why they are produced. With most of the compounds we encounter, however, we do not fully understand what purposes they serve. Fortunately we do not have to understand why they are there, though, to learn whether they can be of use in fighting human disease. Extracting Natural Products During the process of looking for new and useful natural products, the chemists on the expedition will prepare extracts of each of the organisms collected, a process with which you are already familiar if you drink coffee or tea. To make these drinks, you pour boiling hot water over dried and ground-up tea leaves or coffee beans. The hot water "extracts" the various chemicals, including flavoring and natural products such as caffeine, from the dried plant tissue, resulting in a good-tasting and chemically stimulating beverage. On the ship, we prepare extracts of the plants and invertebrates we collect by adding ethyl alcohol and grinding them up in a high-speed blender. We cannot use hot water because some of the compounds present in our samples will not dissolve into it, but they will dissolve into alcohol. Alcohol is also ideal for our purposes because it is safe for chemists and does not damage the cells and enzymes on which we will eventually test the extracts' effects. Once we return to our laboratory at the Harbor Branch Oceanographic Institution, these extracts are used in our biological screening program. At sea, we will analyze the chemicals in them using a technique known as chromatography, which can detect new natural products. Chromatography essentially is the separation of complex mixtures by percolation through a series of substances called adsorbing media that separate the components of the extract mixtures into distinct layers with individual chemical signatures. Two variants of chromatography -- thin layer chromatography (TLC) and high performance liquid chromatography (HPLC) -- will be used to determine if the organisms we have collected contain a variety of chemicals with different signatures. If so, it indicates that they are rich in natural products and hence may harbor something of pharmaceutical potential. We can also determine if different organisms contain the same compounds by determining whether they contain chemicals with the same signatures. When novel compounds are discovered through this process, they are subjected to a battery of tests designed to determine if they might be used to fight certain diseases. For instance, chemicals are applied to cancer cells to see if they kill them. If test results for a compound prove especially promising, then scientists begin a long and complex process of studying the compound to learn more about its chemical structure and how it affects living cells. The full process of discovering a promising new compound at sea, learning how it works, testing its effects on humans with the disease a compound is hoped to fight, and finally getting it on the market as a commercial drug can take 15 years or longer.

© 2005, Harbor Branch Oceanographic Institution