It was previously thought that 5-FU inhibits the enzyme by classical competitive
inhibition. However, it was found that 5-FU is a transition-state substrate, and it forms
a covalent complex with tetrahydrofolate and the enzyme in the same way that the nat-
ural substrate does. The reaction, however, will not go to completion, since the fluoro-
uridine derived from the antimetabolite remains attached to the enzyme, and the latter
becomes irreversibly deactivated. Recovery can occur only through the synthesis of
new enzyme. Fluorouracil is used in the treatment of breast cancer and has found lim-
ited use in some intestinal carcinomas. Unfortunately, this drug has the side effects usu-
ally associated with antimetabolites. Its prodrug, fluorocytosine (8.35, which is also an
antifungal agent) is better tolerated.
8.2.6 Enzyme Targets: Monoamine OxidaseMonoamine oxidase (MAO) (E.C. 1.4.3.4) is an enzyme found in all tissues and almost
all cells, bound to the outer mitochondrial membrane. Its active site contains flavine
adenine dinucleotide (FAD), which is bound to the cysteine of a –Ser–Gly–Gly–Cys–
Tyr sequence. Ser and Tyr in this sequence suggest a nucleophilic environment, and his-
tidine is necessary for the activity of the enzyme. Thiol reagents inhibit MAO. There
are at least two classes of MAO binding sites, either on the same molecule or on dif-
ferent isozymes. They are designated as MAO-A, which is specific for 5-HT (serotonin)
as a substrate, and MAO-B, which prefers phenylethylamine. Similarly, MAO inhibitors
show a preference for one or the other active site, as discussed below.
Monoamine oxidase catalyzes the deamination of primary amines and some secondary
amines, with some notable exceptions. Aromatic amines with unsubstituted α-carbon
atoms are preferred, but aromatic substituents influence the binding of these substrates.
For example,m-iodobenzylamine is a good substrate, whereas the o-iodo analog is an
inhibitor. The mechanism of deamination is as follows: hydrolysis of the Schiff base
that results from loss of a hydride ion on an α-proton yields an aldehyde, which is then
normally oxidized to the carboxylic acid. Aromatic substrates are probably preferred
because they can form a charge-transfer complex with the FAD at the active site, properly
ENDOGENOUS MACROMOLECULES 497