tailieunhanh - Introduction to ENVIRONMENTAL TOXICOLOGY Impacts of Chemicals Upon Ecological Systems - CHAPTER 9

Chuyển dạng sinh học, cai nghiện ma tuý, và phân hủy sinh học GIỚI THIỆU Như đã đề cập ở Chương 5, sau khi nhập cảnh vào một sinh vật sống và di chuyển, một hóa chất nước ngoài có thể được lưu trữ, chuyển hóa, hoặc bài tiết (Hình ). Khi tỷ lệ nhập cảnh lớn hơn so với tỷ lệ trao đổi chất và / hoặc bài tiết, lưu trữ hóa chất thường xảy ra. Lưu trữ hoặc liên kết các trang web có thể không được các trang web của hành động độc hại, tuy nhiên. Ví dụ,. | CHAPTER 9 Biotransformation Detoxification and Biodegradation INTRODUCTION As mentioned in Chapter 5 following the entry into a living organism and translocation a foreign chemical may be stored metabolized or excreted Figure . When the rate of entry is greater than the rate of metabolism and or excretion storage of the chemical often occurs. Storage or binding sites may not be the sites of toxic action however. For example lead is stored primarily in the bone but acts mainly on the soft tissues of the body. If the storage site is not the site of toxic action selective sequestration may be a protective mechanism since only the freely circulating form of the foreign chemical produces harmful effects. Some chemicals that are stored may remain in the body for a long time without exhibiting direct harmful effects. DDT may be considered as an example. Accumulation or buildup of free chemicals may be prevented until the storage sites are saturated. Selective storage limits the amount of foreign chemicals to be excreted however. Since bound or stored toxicants are in equilibrium with their free forms a chemical will be released from the storage site as it is metabolized or excreted. On the other hand accumulation may result in illnesses that develop slowly as exemplified by fluorosis and lead and cadmium poisoning. METABOLISM OF ENVIRONMENTAL CHEMICALS BIOTRANSFORMATION Subsequent to the entry of an environmental chemical into an organism such as a mammal chemical reactions occur within the body to alter the structure of the chemical. This metabolic conversion process is known as biotransformation and occurs in any of several tissues and organs such as the intenstine lung kidney skin and liver. 1999 by CRC Press LLC Phase I Phase II Xenobiotic Primary Metabolite Oxidation Reduction Hydrolysis Conjugation with glycine GSH or glucuronate Secondary Metabolite Figure The two phases of xenobiotic metabolism. By far the largest number of these chemical reactions are .