Cytochrome P450 enzymes reactive intermediates activated

Humans are exposed continuously and unavoidably to a myriad of potentially toxic chemicals that are inherently lipophilic and, consequently, very difficult to excrete. To effect their elimination, the human body has developed appropriate enzyme systems that can transform metabolically these chemicals to hydrophilic, readily excretable, metabolites. This biotransformation process occurs in two distinct phases. Phase I and Phase II, and involves several enzyme systems, the most important being the cytochromes P450. The expression of these enzyme systems is regulated genetically but can be modulated also other factors, such as exposure to chemicals that can either increase or impair activity. Paradoxically, the same xenobiotic-metabolizing enzyme systems also can convert biologically inactive chemicals to highly reactive intermediates that interact with vital cellular macromolecules and elicit various forms of toxicity. Thus, xenobiotic metabolism does not always lead to deactivation but can result also in metabolic activation with deleterious consequences. 

Some components of the immune system have metabolic capability including some forms of the cytochrome P450 system. Therefore, some chemicals, which are inert in their parent form but can be metabolized to reactive intermediates, can be activated by the immune system. However, it is important to emphasize that the metabolic capability of the immune system is minor when compared with other organ systems, most notably the liver, and that it possesses only a small repertoire of metabolizing enzymes. This point is discussed below as a major type of indirect mechanism of immunosuppression. 

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