134
of each FMO enzyme non-covalently binds one mole of FAD present near the active
site; adjacent to this is a second highly conserved glycine-rich region (residues 186
to 213) that binds NADPH. FMO is heat-labile and can be deactivated in the absence
of NADPH by heating microsomes to a temperature of 50 °C for 1 min in contrast
to cytochrome P450, which is deactivated with a non-ionic detergent, such as 1%
Emulgen 911. The pH optimum for FMO-catalysed reactions is 8 to 10, which is
slightly higher than that for most P450 reactions (pH 7 to 8).
The reactions catalysed by FMO include the oxidation of nucleophilic tertiary
amines to N-oxides, secondary amines to hydroxylamines and nitrones, and primary
amines to hydroxylamines and oximes. It also oxidizes several sulfur-containing
xenobiotics (such as thiols, thioethers, thiones, and thiocarbamates) and phosphines
to S- and P-oxides, respectively. With few exceptions, FMO acts as an electrophilic
oxygenating catalyst, which distinguishes it from most other flavoprotein oxidases
and monooxygenases. During the oxygenation of xenobiotics, the
4a- hydroperoxyflavin is converted to 4a-hydroxyflavin with the transfer of the flavin peroxide oxygen to the substrate. The final rate-limiting step in the catalytic
cycle involves dehydration of 4a-hydroxyflavin (which restores FAD to its resting,
oxidized state) and release of NADP
+
. Also this step determines the upper limit of
the rate of substrate oxidation. Binding of NADP
+
to FMO during catalysis is important because it prevents the reduction of oxygen to H 2 O 2 . In the absence of bound
NADP, FMO would function as an NADPH-oxidase that would consume NADPH
and cause oxidative stress through excessive production of H 2 O 2 .
In humans, FMO plays a major role in the biotransformation of several drugs
(e.g. benzydamine, cimetidine, clozapine, guanethidine, methimazole, olanzapine,
sulindac sulfide, tamoxifen and various dimethylaminoalkyl phenothiazine derivatives such as chlorpromazine and imipramine), xenobiotics (e.g. cocaine, methamphetamine, nicotine, tyramine) and endogenous substrates (e.g. trimethylamine,
cysteamine). The major flavin monooxygenase in human liver microsomes, FMO3,
is predominantly involved in the conversion of (S) nicotine to (S)-nicotine N-1oxide, excreted in the urine of cigarette smokers or individuals wearing a nicotine
patch. Therefore, the presence of trans-(S)-nicotine N-1-oxide in urine is clinically
used as an in vivo probe of FMO3 activity in humans. FMO3 is also the principal
Table 5.2 Types of alcohol dehydrogenases (ADH)
Class
Enzyme Function
Location
Class
I
α-ADH
Oxidation of ethanol and other
small aliphatic alcohols.
Liver, adrenal glands, lower levels in
kidney, lungs, blood vessels.
β-ADH
γ-ADH
Class
II
π-ADH
Oxidation of large aliphatic and
aromatic alcohols.
Liver, lower levels in stomach.
Class
III
χ-ADH
Oxidation of long-chain alcohols
and aromatic alcohols.
All tissues, including brain.
Class
IV
σ/μADH
Conversion of ethanol to
acetaldehyde and oxidation of
retinol.
Stomach and other areas of
gastrointestinal tract.
S. Sudhakaran et al.
of each FMO enzyme non-covalently binds one mole of FAD present near the active
site; adjacent to this is a second highly conserved glycine-rich region (residues 186
to 213) that binds NADPH. FMO is heat-labile and can be deactivated in the absence
of NADPH by heating microsomes to a temperature of 50 °C for 1 min in contrast
to cytochrome P450, which is deactivated with a non-ionic detergent, such as 1%
Emulgen 911. The pH optimum for FMO-catalysed reactions is 8 to 10, which is
slightly higher than that for most P450 reactions (pH 7 to 8).
The reactions catalysed by FMO include the oxidation of nucleophilic tertiary
amines to N-oxides, secondary amines to hydroxylamines and nitrones, and primary
amines to hydroxylamines and oximes. It also oxidizes several sulfur-containing
xenobiotics (such as thiols, thioethers, thiones, and thiocarbamates) and phosphines
to S- and P-oxides, respectively. With few exceptions, FMO acts as an electrophilic
oxygenating catalyst, which distinguishes it from most other flavoprotein oxidases
and monooxygenases. During the oxygenation of xenobiotics, the
4a- hydroperoxyflavin is converted to 4a-hydroxyflavin with the transfer of the flavin peroxide oxygen to the substrate. The final rate-limiting step in the catalytic
cycle involves dehydration of 4a-hydroxyflavin (which restores FAD to its resting,
oxidized state) and release of NADP
+
. Also this step determines the upper limit of
the rate of substrate oxidation. Binding of NADP
+
to FMO during catalysis is important because it prevents the reduction of oxygen to H 2 O 2 . In the absence of bound
NADP, FMO would function as an NADPH-oxidase that would consume NADPH
and cause oxidative stress through excessive production of H 2 O 2 .
In humans, FMO plays a major role in the biotransformation of several drugs
(e.g. benzydamine, cimetidine, clozapine, guanethidine, methimazole, olanzapine,
sulindac sulfide, tamoxifen and various dimethylaminoalkyl phenothiazine derivatives such as chlorpromazine and imipramine), xenobiotics (e.g. cocaine, methamphetamine, nicotine, tyramine) and endogenous substrates (e.g. trimethylamine,
cysteamine). The major flavin monooxygenase in human liver microsomes, FMO3,
is predominantly involved in the conversion of (S) nicotine to (S)-nicotine N-1oxide, excreted in the urine of cigarette smokers or individuals wearing a nicotine
patch. Therefore, the presence of trans-(S)-nicotine N-1-oxide in urine is clinically
used as an in vivo probe of FMO3 activity in humans. FMO3 is also the principal
Table 5.2 Types of alcohol dehydrogenases (ADH)
Class
Enzyme Function
Location
Class
I
α-ADH
Oxidation of ethanol and other
small aliphatic alcohols.
Liver, adrenal glands, lower levels in
kidney, lungs, blood vessels.
β-ADH
γ-ADH
Class
II
π-ADH
Oxidation of large aliphatic and
aromatic alcohols.
Liver, lower levels in stomach.
Class
III
χ-ADH
Oxidation of long-chain alcohols
and aromatic alcohols.
All tissues, including brain.
Class
IV
σ/μADH
Conversion of ethanol to
acetaldehyde and oxidation of
retinol.
Stomach and other areas of
gastrointestinal tract.
S. Sudhakaran et al.