Extraction and Isolation of Compounds from Herbal Medicines

Hong-Wei Liu

Plants, particularly medicinal herbs, constitute the foundation of traditional pharma- copeias, and have produced many currently important pharmaceutical drugs, for example, taxol from Taxus brevifolia, vinblastin and vincristine from Catharanthus roseus, and huperzine A from Huperzia serrata. There are at least 250,000 species of higher plants on earth, but only about 5–10% of them have been investigated so far. With the development of modern molecular biotechnology, there is an increasing demand for extraction and isolation of compounds from herbs for the purpose of screening bioactive chemical molecules for new drug development, exploring thera- peutic and preventative mechanism of herbs, as well as establishing quality control and standardization of herbs and herbal products.

The chemicals in plants can be divided into primary metabolites and secondary metabolites based on the range of molecular weight, distributions in species, and biological roles to plants. The ubiquitous macromolecules (mol wt > 2000 amu) of primary metabolism in plants, for example, polysaccharides, proteins, lipids, and nucleic acids, are called primary metabolites. They provide nutrients, and thus are essential for growth and survival. In contrast to primary metabolites, small organic compounds (mol wt < 2000 amu) are produced often by a particular species, usually possessing important biological activity. They are not necessary for growth and survival and are called secondary metabolites.

A number of secondary metabolites in plants serve as chemical messengers and defensive chemicals, and play significant biological and ecological roles. There is growing interest in the study of plant secondary metabolites as they represent a tremendous library of potentially useful leading compounds for new drug develop- ment. The secondary metabolites comprise of a range of chemically diverse compounds. Based on the chemical structure features, they are classified as alkaloids, flavonoids, coumarins, lignans, quinones, terpenoids, and so on.1–5

Isolation of chemical compounds from herbs is an important step for a systematic study of herbal medicine. It provides compounds not only for structural identification or elucidation and standards for quantitative and qualitative analysis for quality control of herbal extracts or products, but also for in vitro bioassay screen and in vivo pharmacological and toxicological study and clinical trials. Before isolation of an herbal medicine, the material should first be identified using methods introduced in Chapter 2, to ensure it is the right species and was collected and stored properly. Extraction and isolation should be carried out with a well-planned scheme based on knowledge of the sample. Different extraction and isolation methods are suitable for different types of compounds. Thus, it is important to do a complete literature search and know the characteristics of compounds in the sample before making the plan. This chapter will mainly introduce the characteristics of major types of secondary metabolites in plants and commonly used methods for extraction and isolation.

Soure: Traditional Herbal Medicine Research Methods; Edited by Willow J.H. Liu; A John Wiley & Sons, Inc., Publication