<prompt> The main enzyme responsible for activation of xenobiotics isCytochrome P - 450Glutathione S - transferaseNADPH cytochrome P - 450 reductaseGlucoronyl transferase  <response>: Cytochrome P450 CYP enzymes in extrahepatic tissues often play a dominant role in target tissue metabolic activation of xenobiotic compounds. They may also determine drug efficacy and influence the tissue burden of foreign chemicals or bioavailability of therapeutic agents. 