Post on 04-Jan-2016
Methylation
• Methyltransferases
• Target: Hydroxyl groups, amines, thiols
• Substrates mainly endogenous: Catechols, noradrenalin, histamine
• Conjugating moiety: Methyl group
• Co-factor: S-adenosylmethionine
S-adenosylmethionine
N
N
NH2
O
OH
CH2S+
CH3
(CH2)2CH
-OOC
H2N
HO
MethylationReaction
OH
OH
OH
O CH3
SAM
Substrate:
Catechol
Enzyme:
Catechol-O-methyltransferase (COMT)
Acetylation
• N-acetyltransferases (NAT)
• Target: Aromatic amines, sulfonamides
• Conjugating moiety: Acetyl group
• Co-factor: Acetyl-CoA
• Few forms: NAT1, NAT2. NAT3: mice
• Genetic polymorphisms: “slow and fast acetylators”
AcetylationReaction
NH2 N CH3
OH
Acetyl CoA
CoASH
C
2-Naphthylamine
2-Aminonaphthalene
2-Acetylaminonaphthalene
2-Acetamidonaphthalene
Non-polar(lipophilic)
Hydrophobic
Lipophobic Hydrophilic(Polar)
XENOBIOTIC
INTERMEDIATE METABOLITE
ELIMINATION
WATER-SOLUBLE METABOLITE
May be reactive/toxic
Can accumulate in tissues
Phase I MetabolismOxidation
Phase II MetabolismConjugation
Sol
ubili
ty in
lipi
ds
Sol
ubili
ty in
wat
er
Phase III (?)Transport into bile, urine
Transporters (Phase III)Organic Anion Transporters
OAT1
OAT2
OAT3
OAT4
Antiport (dicarboxylates eg αKG)
Pump into cell
Organic Anion Transporting Polypeptides
OATP1A1
OATP-A (1A2)
Antiport (eg GSH)
Pump into cell
Organic Cation Transporters
OCT1
OCT2
OCT3
Uniport
Proton
Multidrug Resistance Transporters
MRP1, MRP2
MRP4, MRP6
MDR1, MDR3
ATP pump
Glucurinide, glutathione, sulfate conjugates
Pump out of cell
Lethal Synthesis
Methylation of Mercury: Methylmercury CH3-Hg+
Dimethylmercury CH3-Hg-CH3
http://www.chm.bris.ac.uk/motm/dimethylmercury/dmmh.htm
Glutathione conjugation of 1,2-dihaloalkanesGSH + Br-CH2-CH2-Br GS-CH2-CH2-Br
Ethylene dibromide
Episulfonium ion CH2GS+ CH2
Major reactive species
ElectrophilesEpoxides
(Epoxide hydrolaseGlutathione S-transferase)
Carbonium ionsArylnitrenium ionsEpisulfonium ions
Reactive Oxygen Species
Reactive Oxygen Species (ROS)
• Peroxides– Hydrogen peroxide HOOH– Peroxynitrite OONO-
– Lipid hydroperoxide LOOH
• Free radicals– Superoxide anion O2
•- – Hydroxyl radical HO•
– Nitric oxide NO•
Non-enzymic reaction with anti-oxidants
• Ascorbic acid (Vitamin C)
• alpha-Tocopherol (Vitamin E)
• Glutathione
O
OHOH
O
C HHO
C H
OH
H
O CH3
CH3
H3C
HO
CH3
CH3CH3 CH3
CH3
Superoxide dismutase
Converts superoxide anions to hydrogen peroxide
O2•- + O2
•- + 2H+ O2 + H2O2
Peroxidases
Couple reduction of hydrogen peroxide (or other peroxide) to oxidation of another substrate
(co-oxidation)
ROOH + R’H ROH + R’OH
Peroxidases
• Catalase
• Prostaglandin synthetase
• Myeloperoxidase
• Lactoperoxidase
• Glutathione peroxidase
Glutathione peroxidase
GSH + GSH GSSG
HOOH HOH + HOH
Metabolic Activation/Metabolic Detoxication
• “Metabolism is a double-edged sword”
• Generation of (re)active intermediates
• Detoxication of (re)active intermediates
Pharmacologically active
Chemically reactive
OH O-Glucuronide
OOH
HH OH
S-GlutathioneHH OH
O
H
HO
HOH
Glutathione conjugate
The main purpose of this pilot study is to identify an optimal concentration of diesel exhaust which can be used to study the risks of cardiac changes in healthy older subjects. Results from this study may increase the understanding of how gaseous and particulate air pollutants (which causes the haze seen in some polluted cities) may adversely affect the functioning of the human cardiovascular and respiratory systems. This pilot research will study the responses to diesel exhaust in healthy older subjects with the specific GSTM1 and GSTP1 genotype. For more information, please visit our web site at www.epastudies.org or call Westat EPA Recruiting at 966-0604. Approved June 08, 2007 by the Committee on the Protection of the Rights of Human Subjects Biomedical Institutional Review Board. IRB # 07-0190 (GCRC #2579). This email is sponsored by: U.S. Environmental Protection Agency Human Studies Division located on the UNC-Chapel Hill campus. This email is sponsored by: U.S. Environmental Protection Agency Human Studies Division This email is sponsored by: CEMALB
Another exampleOrganophosphate Insecticides:
• Parathion
• Malathion
Parathion
Malathion
Hydrolysis enzymes
• Serum cholinesterase BChE
• Serum paraoxonase PON1
• Polymorphisms in PON1 – differential sensitivity
Heart diseaseAtherosclerosis Gulf War Syndrome
Effect is the outcome of interaction between susceptibility and
exposure
Target organs
• What makes a particular organ a target for toxicity / infection ?
• What makes a particular organ or species susceptible ?
Portal of entry to Blood to Target Organ
e.g. Intestine to hepatic portal vein to liver to vena cava to heart to lungs back to heart to aorta to rest of body
Location, location, location
Intestines
Hepatic portal veinLiver
Vena cava Aorta
Lungs
Gut flora
• Reductions– nitro to amine
• Hydrolyses– Cleavage of glucuronides
NO NOHNO2 NHH H
Reaction
OHo o
OH
HO
OH
COOH
Glucuronidation
Reaction
OHo o
O H
H O
O H
C O O H
De-glucuronidation-glucuronidase
Aglycone Conjugate
Enterohepatic recirculation(EHC)
Liver
Intestine
• Presence/absence of receptors– Estrogen receptors, Ah receptor
• Presence/absence of transporters/carriers– Resistance to chemotherapy
• Presence of repair mechanisms– DNA repair
• Balance of metabolic activation/detoxication
Factors affecting xenobiotic metabolism
• Intrinsic– Species, strain, gender, age, genotype
• Physiological status– Temperature, time of day, season, – Health status, disease, stress– Diet, nutritional status
• Related to exposure– Route of administration, frequency and size of
dose, co-exposures (induction, inhibition)
Genetic polymorphisms
• CYP2D6 Debrisoquine hydroxylation (poor and extensive metabolizers)
• Acetylation (fast and slow acetylators)
• GSTM null genotype
Changes in P450 levels with ageRats
2A1
2C6
3A2
M: 2C6, 2C11, 3A2
F: 2A1, 2C6, 2C12