Drug Metabolism 2
Steven R. Tannenbaum
20.201
September 2013
Conjugation Reactions
‡ “Phase II” biotransformations attach a hydrophilic
moiety onto drug/metabolite. The resulting product is
too polar to re-cross membranes back into tissues and is
readily excreted.
‡ These type of reactions most often abolish biological
activity
Chemistry
‡ Unlike P450, Phase II reactions are concerted reactions that
involve an electrophile (Y), a nucleophile (X) and a leaving group
(L)
5
;+ <
/
5
;+ <
/
5
; < /+
‡ These reactions involve a pair of electrons, i.e., no radical
intermediate
‡ Electron donating groups enhance nucleophilicity of substratefavors reaction whereas, electron withdrawing groups decrease
nucleophilicity-hinders reaction
Types of Conjugation Reactions
‡ Glucuronidations (UGTs)
‡ Sulfations (STs)
‡ Glutathione transferases (GSH, GST)
‡ Methylations
‡ Acetylations
‡ Amino acid conjugations
Uridine Diphosphoglucuronic Acid Transferases (UGT)
‡
‡
‡
‡
‡
UGTs are a family of enzymes, with two groups (UGT1A and UGT2B)
Found in same tissues as P450, microsomal
Uses uridine-5’-diphospho-D-D-glucuronic acid (UDPGA) as a cofactor
One of the most common conjugative reactions
Catalyze the glucuronidation of electron-rich nucleophilic
heteroatoms/sites
±
±
±
±
±
±
±
Alcohols (ROH)
Phenols (phenolic hydroxy groups, Ar-OH)
Carboxylic acids (R-COOH)
Aromatic Amines (Ar-NH2 or Ar-NHR)
Tertiary amines (R3-N)
Sulfhydryl groups (R-SH)
Carbons w/ sufficient nucleophilicity
Glucuronidation Reaction
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Xenobiotics that are Glucuronidated
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Sulfotransferases (ST)
‡ STs are a family of enzymes with five members (SULT1-5,
SULT1 and 2 are most important in the metabolism of drugs
‡ Found in liver, kidney, intestine, in cytosol
‡ Catalyze the sulfation of
±
±
±
±
Alcohols (R-OH)
Phenols (main group of substrates, Ar-OH)
Arylamines (Ar-NH2)
N-hydroxy compounds (R-NH-OH)
‡ Cofactor (3’-phosphoadenosine-5’-phosphosulfate, PAPS) is in
limited supply, drug can overwhelm the system
‡ Low activity in some individuals thought to be the cause of
wine induced headache
Sulfation Reaction
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Xenobiotics that are Sulfated
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Biological Roles and Cycle of Glutathione
cys + glu
Degradation
and
transport
cys
Synthesis
γ-glu-cys
gly
Coenzyme
functions
Protein
GSH
+
NADP
GSH Thioltransferases
GSSG
Peroxidases
Reductase
Reactions with
ROS/RNS
GSH S-Conjugates
Protective functions
NADPH,H+
GSSG
Oxidation reduction
Image by MIT OpenCourseWare.
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Glutathione Transferases (GST)
‡ Family of enzymes (2 microsomal and 7 cytosolic)
‡ Referred to as an “electrophile killing” enzyme
‡ Uses reduced glutathione (GSH) as a cosubstrate ; Cysteine sulfhydryl
group is the nucleophilic (attacking) moiety
‡ Glutathione can exist in a reduced form (GSH) or can be oxidized to a
dimer (GSSG) via a disulfide bond
‡ GST catalyze the reaction of GSH with electrophiles
± GSH is reactive on its own; the enzyme holds the substrate in place for an
increased reaction rate
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Glutathione Transferase Reaction
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Important Points
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Cl
O
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Cl
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Cl
M
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M
M
2,3,7,8-Tetrachlorodibenzo-p-dioxin
Indolo(3,2-b)carbazole
CH3
Benzo[a]pyrene
3-Methylcholanthrene
Structures of aromatic hydrocarbon (AH) receptor ligands. Examples of toxic
halogenated aromatic hydrocarbons (2,3,7,8-Tetrachlorodibenzo-p-dioxin).
Carcinogenic nonhalogenated polycyclic aromatic hydrocarbons (3-Methylcholanthrene and Benzo[a]pyrene) and dietary constituents (Indolo(3,2-b)
carbazole) are shown.
AHR
Ligand
AHR
Hsp90
Hsp90
DME
Hsp90
Hsp90
Transformation
Hsp90
Hsp90
AHR
ARNT
ARNT
AHR
ARNT
AAA
AAA
AHRE
mRNA
Endobiotic
and xenobiotic
merabolism
AAA
N
Nucleus
Protein
Cytoplasm
AHR = Aromatic Hydrocarbon Receptor
DME = Drug Metabolizing Enzyme
AHRE = AH-responsive-element
ARNT = AH-receptor-nuclear-translocator
Hsp90 = Heat shok protein 90
Image by MIT OpenCourseWare.
Nuclear
receptor
AhR
CAR
PXR
Uptake
transporter
Export
transporter
CYPs
UGTs
CYP1A1
UGT1A1
CYP1A2
UGT1A6
(rUGT1A7)
OATP2
CYP2B6
UGT1A1
MRP2
(mOatp2)
(rCYP2Bl)
(mCyp2b10)
(rUGT2B1)
(mMrp2)
OATP2
CYP3A4
UGT1A1
MRP2
(mOatp2)
(rCYP3A23)
(mUgt1a6)
(mUgt1a9)
MRP3
n.d.
(mCyp3a11)
n.d.*
Inducible rodent enzymes are listed in parenthesis.
*n.d., not detected.
Selected human and rodent CYPs, UGTs and glucuronide transporters induced
by the Ah receptor. CAR and PXR.
Image by MIT OpenCourseWare.
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hemistry to itigate the isk of diosyncratic rug oxicity: erspective ased on the ritical xamination of rends
in the op 200 rugs arketed in the United States." &KHPLFDO5HVHDUFKLQ7R[LFRORJ\ 24, no. 9 (2011): 1345-410.
Published in: Antonia F. Stepan; Daniel P. Walker; Jonathan Bauman; David A. Price; Thomas A. Baillie; Amit S. Kalgutkar; Michael D. Aleo;
Chem. Res. Toxicol. 2011, 24, 1345-1410.
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Note that activation of BHT to a toxic quinone methide can be catalyzed by
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MIT OpenCourseWare
http://ocw.mit.edu
20.201 Mechanisms of Drug Actions
Fall 2013
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