Figure 8-2 Proportion of drugs metabolized by major phase 1 or phase 2 enzymes. The relative size of each pie section indicates the estimated percentage of phase 1 (left) or phase 2 (right) metabolism that each enzyme contributes to the metabolism of drugs based on literature reports. Enzymes that have functional allelic variants are indicated by an asterisk. In many cases, more than one enzyme is involved in a particular drug's metabolism. CYP, cytochrome P450; DPYD, dihydropyridine dehydrogenase; GST, glutathione S-transferase; NAT, N-acetyltransferase; ST, sulfotransferase; TPMT, thiopurine methyltransferase; UGT, UDP-glucuronosyltransferase. (Adapted from Wilkinson G: Pharmacokinetics: The dynamics of drug absorption, distribution and elimination. In Hardman JG, Limbird LE, Goodman GA [eds]: Goodman and Gilman's the Pharmacological Basis of Therapeutics, 10th ed. New York, McGraw-Hill, 2001.)