The graph on the left lists the major isoforms of CYP450 and their relative roles in drug metabolism (not relative amounts found in the liver) based upon the number of drugs that are known to be metabolized by that particular isozyme. CYP3A is responsible for the metabolism of the largest number of drugs followed by CYP2D6.
The graph on the right summarizes the relative quantity of specific P450 families found in the liver.1The CYP3A family is present in the largest amounts. CYP2D6 accounts for less than 2% of the total content of P450 in the liver, but as shown on the left, is responsible for the metabolism of a large fraction of drugs. A large amount of cytochrome P450 has not yet been characterized.