Table 1. Summary of Percentages of Metabolism Attributed to Enzymes^a.

enzymes	all chemicals	general chemicals	natural and physio- logical chemicals	drugs	marketed drugs	drugs in development
FMO	2	3	1	2
MAO	1	2	1	1
AKR	2	3	4	1
other (non-P450)	<1	<1	<1	<1
P450s
1A1	7	11	7	5	4	4
1A2	10	15	8	9	8	8
1B1	3	6	4	1	1	1
2A6	4	5	4	2	3	2
2B6	5	6	3	4	5	4
2C8	8	3	3	5	6	5
2C9	4	7	8	10	10	10
2C19	9	6	6	9	9	8
2D6	10	8	6	13	14	12
2E1	5	8	4	3	4	3
3A4	20	13	15	27	22	29
3A5	4	2	3	6	6	6
other P450s	11	10	29	6	8	8

^aPercentages are derived from Figures 1–4. The percentages presented in the table (and Figures 1–4) are based on the number of reactions rather than the number of compounds because many chemicals can undergo multiple reactions.