Drug Metabolism in Drug Design and Development Basic Concepts and Practice

although the range of substrates is much more restricted. The substrates are
generally soft nucleophiles, almost always N, S, and P atoms (phosphines in the
case of P). Of the reactions ascribed to P450 (Figs. 2.3 and 2.4), FMO is
generally restricted to heteroatom oxygenation (Fig. 2.3).
Five forms of FMO are found in humans (and several experimental
animals). Important physiological substrates have not been established,
although some possibilities have been proposed (Ziegler, 1993). An interesting
polymorphism involves defects in FMO 3, giving rise to deficient metabolism
of trimethylamine and a resulting ‘‘fish-odor syndrome’’ (Al-Waiz et al., 1987).
In contrast to P450, FMOs appear not to be inducible or readily inhibited.
FMO is a single-protein mixed-function oxidase, in contrast to P450. The
flavin (FMN) acts as both the entry point for electrons and the terminal
oxidant, using a C-4a-hydroperoxide. The catalytic mechanism (Fig. 2.5)
provides some insight into the properties of this enzyme. The general ‘‘resting
state’’ has the 4a-hydroperoxide ready to react with substrates, and the rate-
limiting step is generally the breakdown of the flavin 4a-alcohol (step involving
H 2 O release in Fig. 2.5). The manifestation of this mechanism is thatkcat(Vmax)
does not vary considerably butKmdoes, due to rates of reactivity more than
substrate affinity.
A practical consideration in working with tissues is that FMOs are not very
heat stable, particularly in the absence of pyridine nucleotides (Ziegler and

+•

+ RCHO

+

OR OR + HCO 2 H

OR

O

OH

O

R 2

R 1 R 1

R 2

O CH 3 O

N

H

N

NH 2

N

N

NH 2

R

O

H R

RC N

X OH HO OH

R 1 R 2

R 3

OH

R 2

R 1

R 3

OH

RCH=N-OH

-X-

FIGURE 2.4 Some uncommon P450 oxidations (Guengerich, 2001).

OXIDATION ENZYMES 21