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Figure 9. The catabolic pathway of 5-fluorouracil (5FU). The majority of this process occurs in the liver and the rate-limiting step is the conversion of 5FU to dihydrofluorouracil by the enzyme dihydropyrimidine dehydrogenase. The principal catabolite is 
-fluoro-
-alanine that is predominantly excreted in urine. The species carboxy-fluoro-beta alanine (carboxy-FBAL) is rarely encountered since its formation is highly pH dependent. It would normally form under acidic conditions, which is rarely found in the liver and almost never in bile.
