Where Do We Find Natural Products?
Natural products can be extracted from the cells, tissues and secretions of microorganisms, plants and animals. A crude (unfractionated) extract from either of these sources will contain a range of chemical compounds of diverse and often novel structure. Chemical diversity in nature is based on biological diversity, so researchers travel the world to obtain samples for analysis and evaluation in biological or drug discovery tests. This effort to research natural products is known as bioprospecting.
The discipline of pharmacognosy, which is the study of biologically active natural products, provides the necessary tools to identify, select and treat natural products for medical use. Usually, a natural extract has some form of biological activity that can be detected and attributed to a single compound or a set of related compounds produced by the body. These active compounds can be used directly as they are in drug discovery and development, or they can be synthetically modified to improve biological properties or reduce side effects. Examples of biological sources used to find new natural products are described below.
Prokaryotic organizations
A prokaryote is a single-celled organism that does not have a membrane bound nucleus (karyon), mitochondria or any other membrane bound organelle. The word prokaryote comes from the Greek πρό "pro" and καρυόν (karyon) "nut" or "kernel". Prokaryotes can be divided into two domains, Archaea and Bacteria. On the other hand, species with nuclei and organelles (animals, plants, fungi and protists) are placed in the Eukaryota domain.
in prokaryotes, all water-soluble intracellular components (proteins, DNA and metabolites) are located together in the cytoplasm enclosed by the cell membrane, rather than in separate cell compartments. Prokaryotes are also much smaller than eukaryotic cells.
Bacteria
Generally a few micrometers long, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth and are present in most of its habitats. Bacteria inhabit soil, water, acid hot springs, radioactive waste and deep parts of the earth's crust. Bacteria also live in symbiosis and in parasitic relationship with plants and animals. Most bacteria have not been characterized and only about half of the bacterial phyla has species that can be cultured in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology. There are typically 40 million bacterial cells in one gram of soil and one million bacterial cells in one millilitre of fresh water. Bacteria are an important source of natural products. Figure 6.4 shows some examples of natural bacterial products that have had an impact on our society, including several antibiotics.
The incidental discovery and subsequent clinical success of penicillin has prompted large-scale research into other environmental microorganisms that could produce natural anti-infective products. Soil and water samples were collected from around the world, which led to the discovery of streptomycin (derived from the bacteria Streptomyces griseus) and the finding that bacteria, not just fungi, are an important source of natural antibacterial products. This, in turn, has led to the development of an impressive arsenal of antibacterial and antifungal agents including amphotericin B, chloramphenicol, erythromycin, neomycin B, daptomycin and tetracycline (all from Streptomyces spp.), polymyxins (from Paenibacillus polymyxa) and rifamycins (from Amycolopsis rifamycinica).
Although most drugs derived from bacteria are used as anti-infectives, some have found their application in other areas of medicine. Botulinum toxin (from Clostridium botulinum) and bleomycin (from Streptomyces verticillus) are two examples. Botulinum toxin is the neurotoxin responsible for food poisoning by botulism. It is caused by the bacterium Clostridium botulinum, which can develop in poorly sterilized canned meat and other canned foods. Poisoning can be fatal depending on the amount of toxin ingested. It causes muscle weakness and paralysis. This toxin is now used in cosmetics to help reduce facial wrinkles. It is injected in small doses into areas such as the forehead to paralyze the muscles that create wrinkles. In addition, bleomycin glycopeptide is used to treat several cancers including Hodgkin's lymphoma, head and neck cancer and testicular cancer. New trends in the field include metabolic profiling and isolation of natural products from new bacterial species present in under-explored environments. For example, the discovery of secondary metabolites from symbiotes or endophytes. Symbionts are organisms that live in close association with another, often larger, organism called the host. Endophytes are non-weedy symbionts that are associated with plants for at least part of their life cycle. In addition, the discovery of organisms from tropical environments, underground bacteria found in the underground depths by mining and drilling, and marine bacteria continue to add to the complexity of the secondary metabolites discovered.
Archaea
The discovery of organisms now classified as Archaea is quite recent in our history, dating back to 1977 by researchers Carl Woese and George E. Fox. Genetic sequencing has been used to show that a distinct branch of ancient prokaryotic organisms diverged at an early stage in the history of life on Earth. Thus, Woese suggested dividing prokaryotic organisms into two broad categories, bacteria and archaea, according to these genetic differences. It should be noted that many Archaea have adapted to life in extreme environments such as polar regions, hot springs, acid springs, alkaline springs, salt lakes, and high pressure deep ocean waters. These species of Archaea are known as extremists.
Before the discovery of Woese and Fox, scientists thought that prokaryotic extremists were bacteria from common bacterial species that are more familiar to us. Now, the evidence suggests that these are actually very ancient life forms, and that they can have strong evolutionary links with the first life forms on Earth. Woese's work on Archaea is significant in its implications for the search for life on other planets, as extremophiles can be robust enough to exist in extreme environments located on distant worlds. Because many Archaea have adapted to life in extreme environments, they also possess enzymes that are functional under quite unusual conditions. These enzymes are potentially useful in the food, chemical and pharmaceutical industries, where biotechnological processes often involve high temperatures, extreme pH, high salt concentrations and/or high pressures.
For example, Pyrococcus furiosus is an extremophile species of Archaea. It can be classified as hyperthermophilic because it thrives better at extremely high temperatures - higher than those preferred by a thermophile. It is distinguished by an optimal boiling water growth temperature of - 100°C (a temperature that would destroy most living organisms). Recently, Dr. Tang's research group isolated a thermostable enzyme from this species that can break down lactose, a disaccharide sugar found in milk (Figure 6.6). Lactose intolerance is a common health problem that causes gastrointestinal symptoms and avoidance of dairy products in people with lactose intolerance. Since milk is a primary source of calcium and vitamin D, people with lactose intolerance often obtain insufficient amounts of these nutrients, which can have adverse health effects. The production of lactose-free milk can provide a solution to this problem, although it requires the use of lactase of microbial origin and increases the risk of contamination. The use of thermostable lactase enzymes can solve this problem by operating under pasteurization conditions. Initial explorations of this enzyme show that it has optimal activity on 100 celsius degree and that it is thermostable even on 110 celsius degree.
Eukaryotic organisms
The eukaryotic organisms include four major kingdoms: Protista, Fungi, Plantae and Animalia. Fungi are heterotrophic, unicellular or multicellular eukaryotic organisms that decompose mainly in the environment. Heterotrophs are organisms that cannot produce their own food. Plants are multicellular eukaryotic organisms that are autotrophic or capable of producing their own food. Plants are also characterized by real roots, stems and leaves. Animals are multicellular, eukaryotic, heterotrophic organisms, characterized by their mobility at some point in their lives. The term Protista (or sometimes Protoctista) is still often used to describe all the other Eurkaryotic organisms that do not enter the fungi, plantations or animals kingdom. However, this is not an ideal group, as there are protists who resemble animals, plants and fungi grouped under the same generic term. Many scientists prefer to reclassify the protist kingdom into sub-groups of related organisms from phylogenetic data, rather than using the old protist classification. In fact, the phylogenetic classification proposed by Carl Woese divides the Protista Kingdom into three main groups: ciliates, flagellates and microsporidia. In the next section, we will focus on examples of natural products from the realms of mushrooms, plants and animals. However, remember that many protists are also producers of interesting natural products.
Mushrooms
As mentioned above, fungi are heterotrophic eukaryotic organisms that decompose mainly in the environment. They include unicellular organisms such as yeast and mould, and multicellular organisms that have a fruiting body, such as fungi. Mushrooms produce a myriad of secondary natural products. Some are very toxic and have given rise to common names such as body cap, destructive angel and button mushroom. Others have found great use in medicine. For example, several anti-infective drugs have been derived from fungi, including penicillins and cephalosporins (Penicillium chrysogenum antibacterials and Cephalosporium acremonium, respectively) and griseofulvin (a Penicillium griseofulvum antifungus). Another medically useful fungal metabolite is lovastatin (from Aspergillus terreus), which has become a precursor to statins, a series of drugs commonly used to lower cholesterol levels.
Ergometrine (from Claviceps spp.) acts as a vasoconstrictor and is used to prevent bleeding after delivery. You will notice in the photo of Claviceps spp. that this type of fungus commonly grows on cereal crops such as wheat and barley. Contamination of cereal crops with this fungus can lead to human poisoning if large quantities of fungi are consumed. This type of poisoning is known as ergotism and can cause convulsions. The vasoconstrictor properties of ergometrine can also cause gangrenous side effects when ingested in toxic doses. The more poorly vascularized distal structures such as fingers and toes are affected first. This can lead to loss of peripheral sensation, edema and, ultimately, death and loss of affected tissues.
Cyclosporine is another amazing example of a fungal metabolite with significant medical implications. Cyclosporin is an alkaloid structure composed of amino acid building blocks that form a cyclic peptide structure. Its main biological activity is to suppress the immune response. Thus, it is widely prescribed to patients undergoing organ transplantation, to help reduce the risk of organ rejection. Cyclosporine was isolated in 1971 from the fungus Tolypocladium inflatum. After 12 years of laboratory studies and clinical trials, its use was approved by the FDA in 1983. It is on the World Health Organization's list of essential medicines as one of the most effective and safest medicines a health system needs. It should be noted that T. inflatum is the unicellular asexual form of a fungus that can also take a sexually reproductive multicellular life stage, where it is known as the fungus Cordyceps subsessilis. Cyclosporine is produced only at the asexual stage of the body's life, demonstrating that gene expression can vary considerably within an organism due to the stage of life or other factors present in the organism's environment.
Plants
Life forms that are classified in the plant kingdom are multicellular eukaryotic organisms that are autotrophic or capable of producing their own food. They produce their own food through the process of photosynthesis, where they use the sun's light energy to convert carbon dioxide and water into simple sugars. Oxygen is a by-product of this reaction. Thus, plants are a major source of oxygen on the planet. It is estimated that there are about 250,000 to 300,000 different plant species on the planet. In addition to producing oxygen and being used as a food source, plants are also an important source of complex and structurally very diverse secondary metabolites. This structural diversity is attributed in part to the natural selection of organisms that produce powerful compounds to deter herbivores (food deterrents). Although the number of plants studied in depth is relatively small, many pharmacologically active natural products have been identified and are currently used as medical treatments. Clinically useful examples include the anti-cancer agents paclitaxel and vinblastine (from Taxus brevifolia and Catharanthus roseus, respectively), the antimalarial agent artemisinin (from Artemisia annua), morphine (from Papaver somniferum), an opioid analgesic drug, galantamine (from Galanthus spp.), used to treat Alzheimer's disease.
Animals
Animals are multicellular and eukaryotic organisms of the Animalia kingdom. As described above, animals are heterotrophic organisms and are characterized by their mobility at some point in their lives. Animals can be divided into vertebrates and invertebrates. Vertebrates have a vertebral column and represent less than five percent of all described animal species. These include fish, amphibians, reptiles, birds and mammals. The remaining animals are invertebrates, which do not have a backbone. These include molluscs (clams, oysters, octopi, squid, squid, squid, snails); arthropods (centipedes, centipedes, insects, spiders, scorpions, crabs, lobsters and shrimps); anelids (earthworms, leeches), nematodes (wireworms, hookworms), flatworms (tapeworms, liver flukes), cnidarians (jellyfish, sea anemones, corals), ctenophores (comb jellies) and sponges. The study of animals is called zoology.
Animals are also a source of natural bioactive products. In particular, poisonous animals such as snakes, spiders, scorpions, caterpillars, bees, wasps, millipedes, ants, toads and frogs have attracted much attention. Indeed, the components of venom (peptides, enzymes, nucleotides, lipids, biogenic amines, etc.) often have very specific interactions with a macromolecular target in the body. As with plant food deterrents, this biological activity is attributed to natural selection, with organisms capable of killing or paralyzing their prey and/or defending themselves against predators being more likely to survive and reproduce.
For example, chlorotoxin is a 36 amino acid peptide found in the venom of the killer scorpion (Leiurus quinquestriatus) that blocks the channels of low-conductance chloride. He uses this toxin to immobilize his prey.
Remarkably, in humans, chlorotoxins bind preferentially to brain cancer cells with glioma. A glioma is a type of tumour that forms in the brain or spinal cord. It can often become malignant, a term used to describe a cancer with a poor prognosis that is likely to spread to different parts of the body. Remarkably, chlorotoxin binds only to tumour cells and not to normal brain tissue. This characteristic has led to the development of new methods for treating, diagnosing and eliminating different types of cancer. For example, TM-601, which is the synthetic version of chlorotoxin, is currently in a Phase II clinical trial. Radioactive iodine 131 can be attached to TM-601 and used to treat malignant glioma. TM-601 crosses blood-brain and tissue barriers and binds to malignant tumour cells in the brain without affecting healthy tissues. When TM-601 is attached to radioactive iodine 131, iodine is also recruited specifically for the tumor where it can preferably kill tumor cells.
In addition, researchers at the Fred Hutchinson Cancer Research Center have also created a chlorotoxin derivative called BLZ-100 that is attached to a fluorescent dye. This provides a durable signal that makes the tumor shine, almost as if all parts of the tumor had been painted. It can be used in real time to help a surgeon determine where the edges of the tumor are or where the tumor has spread, so that it can be completely removed. Animal tests with this "tumor paint" have shown positive results with many types of cancer. The tumour that was removed from a dog with breast cancer - it's called breast carcinoma in a dog - and the surgeons were aware of that big dot at the bottom right, it was cancer, but that's all they could say on the clinical examination and the ultrasounds that had been done in advance. This dog received a dose of tumor paint the day before the operation. Not only could they see the main tumor, but they could also see other areas of cancer that were not visible to the naked eye.
Other new drugs derived from animal venoms include teprotide, a peptide isolated from the venom of the Brazilian viper Bothrops jararaca. Teprotide has been shown to have antihypertensive activity and has provided an initial lead compound for the development of blood pressure lowering drugs. It was not a good drug candidate on its own, due to isolation costs and lack of oral availability. However, the structure was used as the lead compound and many derived structures were created to try to find smaller, more soluble and more orally active compounds that had the same biological activity. This resulted in the development of the currently prescribed antihypertensive agents, cilazapril and captopril.
In addition to the terrestrial animals described above, many marine animals have been examined for pharmacologically active natural products, including corals, sponges, tunicates, sea snails and bryozoans producing chemicals with interesting analgesic, antiviral and anti-cancer activities. Two examples of clinical use have been developed: ω-conotoxin (from the marine snail Conus magus) and ecteinascidin 743 (from the tunicate Ecteinascidia turbinata). The first, ω-conotoxin, is used to relieve severe and chronic pain, while the second, ecteinascidin 743, is used to treat cancer.
Medicines from the sea. In the upper panel, the sea snail, Conus magnus and its active metabolite, ω-conotoxin, are shown. Note that ω-conotoxin is a protein. Thus, its structure is far too large to show all the organic links. Proteins are often represented in ribbon diagrams to give you an idea of the three-dimensional folding patterns. In the lower panel, the tunicate turbinate Ecteinascidia and its metabolite, ecteinascidine 743 (ET-743) are shown. Photo of Conus magnus provided by: Richard Parker. Ribbon diagram of ω-conotoxin provided by : Fvasconcellos. Photo of the tunicator, Turbine Ecteinascidia provided by : Sean Nash.
