Extraction, Isolation, and Identification of Compounds in Herbal Medicines

1.2.1 Extraction, Isolation, and Identification of Compounds in Herbal Medicines

All the substances in the universe, including plants, are composed of chemical compounds. To study herbal medicine, the major bioactive chemical components should be first known. Only after the biological compounds in herbs are correctly extracted, isolated, and identified can biochemical, biological, or pharmacological studies be performed scientifically.

Chemical studies of herbal medicines provide fundamental substances for further studies of biological and pharmacological activity. During the earlier decades of the 1800s, chemical studies in plants could only be performed on active compounds that were highly concentrated and isolated into a relatively pure form by techniques such as distillation or extraction with water, acid, base, or alcohol. Their structures were mainly determined by chemical degradation and proven by synthesis in an unambiguous manner. Scientists were unable to determine the stereochemistry of compounds.

The well-known example is the story of aspirin. According to records about willow leaves as an antipyretic treatment in Ebers papyrus, and following the same application of teas made from willow bark as an English herb, chemists and phar- macists successfully isolated salicin from the bark of the white willow, Salix alba, between 1825 and 1826. The compound responsible for the remedy was subsequently converted to salicylic acid via hydrolysis and oxidation, and proved as such a successful antipyretic (fever reducer) that it was actively manufactured and used worldwide. Due to severe gastrointestinal toxicity, salicylic acid was converted into acetylsalicylic acid via acetylation by scientists at Bayer. It was given its trade name of aspirin in 1899. Today, aspirin is still the most widely used analgesic and anti- pyretic drug in the world.

Since the 1950s, chromatography, including medium-pressure liquid chroma- tography (MPLC) and high-performance liquid chromatography (HPLC), and other methods such as supercritical fluid extraction (SFE), droplet countercurrent (DCC), and high-speed countercurrent (HSCC) have been popularly applied for isolation of natural products, while different types of spectral equipment such as infrared (IR), ultraviolet (UV), nuclear magnetic resonance (NMR), circular dichroism (CD), and mass spectrometer (MS), as well as MS coupled with gas chromatography (GC), have been commonly used for structure identification. Later on, LC-MS and LC-NMR also became available and gradually more popular in the last few decades. These advances have made the time for extraction, isolation, and identification of compounds from herbal medicines much shorter than that of a century ago. Modern extraction and isolation techniques, combined with all types of chromatography, are often guided by bioassays to isolate the active compounds. High-throughput screening with robots also dramatically lowers the screening times. Thus, structure-efficacy elucidation of newly isolated bioactive compounds is no longer a time-consuming and difficult process.

However, the process of finding new drug candidates from herbs for drug development is no longer as easy as the story of aspirin. The story of taxol is that of a difficult journey of a trace compound from a plant becoming a powerful new drug. Taxol is one of the most well-known diterpenes with a very complex steroid structure and anticancer activity. The extract of the bark of Pacific yew (Taxus brevifolia) was first found to be cytotoxic in a cellular assay in 1964. The active ingredient was isolated in 1966 with a very low amount, and the structure was published in 1971. By 1969, 28kg of crude extract had been isolated from almost 1200 kg of bark, but yielded only 10 g of pure material. The research result showed that it acts to stabilize the mitotic apparatus in cells, causing them to act as normal cells rather than undergo rapid proliferation as they do in cancer. But it was not until the late 1980s that its value as an anticancer drug was confirmed.1

Current modern methods and techniques such as all kinds of chromatography and spectrometry, and their combined application make the extraction, isolation, and structure identification of bioactive compounds from herbs dramatically faster than half a century ago. Highly accurate analytical equipment, such as HPLC coupled with UV and/or MS and other detectors, makes the quality control and standardization of herbal products more reliable for pharmacological and clinical studies. Advanced biochemical and biological technologies, such as microarray, allow scientists to easily explore the mechanism study at the enzyme, receptor, and gene levels quantitatively using only small amounts of samples. These advanced technologies and their applications to herbal study will be introduced in the following chapters. With all these available high technologies, time for isolation and identification of compounds from herbs is becoming shorter and trace bioactive compounds are more easily obtained. With the popularity of various spectroscopy methods, identification of isolated compounds is becoming much easier than it was decades ago. Application of hyphenated LC-UV/MS and LC-NMR techniques greatly accelerates the systematic identification of compounds in an herbal extract.

To perform any herbal study, identification of the herbal materials used for study should never be neglected. Morphological, microscopic, physical, or chemical iden- tification can all be applied to identify the raw materials. The availability of HPLC chromatogram or gene fingerprints makes identification of species highly accurate.

Soure: Traditional Herbal Medicine Research Methods, Edited by Willow J.H. Liu Copyright © 2011 John Wiley & Sons, Inc.

REFERENCES

1. Goodman, J. and Walsh, V. (2001) The Story of Taxol: Nature and Politics in the Pursuit of An Anti-Cancer Drug. Cambridge, Cambridge University Press.