ABSTRACT
Endogenous compounds, such as steroid hormones, are metabolized in the cell, either for activation or deactivation purposes. These biotransformation processes are catalyzed by so-called cytochrome P450 enzymes, a diverse class of enzymes belonging to the mixed-function oxygenase (MFO) system. Environmental pollutants, such as insecticides and other xenobiotics, are also metabolized by the same enzymatic system. Metabolism of these compounds is directed to deactivation and excretion. In view of the great variety of xenobiotics degraded by the MFO system, this system is also highly specialized in insects. Depending on the type of substrate and cellular microenvironment, the enzymes exhibit a mono-oxygenase, an oxidase, or a substrate reductase activity. Cytochrome P450 enzymes in insects are found in several important tissues, such as the digestive tract, the Malpighian tubules, and the fat body. Other cellular defense systems provide a cooperative response, and the MFO system can therefore be considered a member of the cellular defense system.
