Carboxlesterases (EC3.1.1.1 ) can be inhibited by combining with organophosphorus compounds and classified into B-esterase.The enzymes are responsible for the hydrolysis of ester, thioester and amide bonds of many compounds although their natural substrate and physiological role are nuclear. Recently, it is found that purified carboxylesterase may play an important physiological role in biotransformation of drugs or xenobiotics and lipid metabolism.Human carboxylesterases consist of an important family of isoenzymes that have distinct biochemical characteristics with studies on the primary structure and gene sequence of many different isoenzymes.