The more soluble versions of these compounds can then be dissolved in the blood and transported to the kidneys for elimination. By solubilizing xenobiotics, the cytochromes enable the compounds to dissolve readily in the blood, which in turn leads to their efficient removal by the kidneys. Another important property of the cytochrome P450 enzymes is that they are inducible, that is, the genes that make them can be turned on and off by the same alien molecules they act on. At this point, the cytochrome enzyme's genes are activated and large amounts of enzymes are made. Then when the cytochromes have modified and brought about the elimination of the intruder, the genes revert to a state of lowered activity and the levels of cytochromes in the cells drop.
Some foreign compounds can do the opposite and turn off cytochrome genes.
Natural xenobiotics rarely do this but many drugs can, since, when one takes an evolutionary perspective, they are substances most organisms have never experienced. A powerful example of this is the activity of the HIV protease inhibitor ritonavir. This drug inhibits the synthesis of some cytochrome P450 enzymes and thus to a significant extent escapes chemical modification and elimination. They efficiently detoxify low doses of pesticides and both natural and human-made carcinogens. However, some xenobiotics are made more dangerous through the action of cytochrome P450 enzymes.