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Environmental Health and
Toxicology
Chapter 3 Metabolism of Xenobiotics
(foreign compounds)
外来物 的 新陈代谢
Phases of Metabolism
Once xenobiotics enter our body, most will be
excreted 排泄 or metabolic inactivation 灭活 as quick
as possible.
Water-soluble xenobiotics will be excreted through
the kidneys 肾 as urine 尿，no metabolism required.
Lipophilic亲脂性xenobiotics will be excreted by
biliary excretion 胆汁中的排泄 or undergo
metabolism to become more polar極性 and then
water-soluble and excreted by the kidneys.
Two phases in the metabolism of xenobiotics
1. Biotransformation 生物转化 which involves
oxidative 氧化 reactions
2. Conjugation reaction 结合反应 which involves
combination with highly water-soluble moieties一
部分或一半
Biotransformation of xenobiotics might release some
highly reactive compounds高活性的化合物 such as
free radicals自由基, strong electrophiles 亲电体 or
highly stressed three-member rings (epoxide 环氧化物,
azaridine 楝碱, episufide and diazomethane 重氮甲烷;)
which will undergo nucleophilic 亲核的 ring opening.
Freely roaming reactive compounds are not
welcome in a living organism because they react
randomly with macromolecules巨大分子,高分子
such as DNA, RNA, and proteins. The reactions
might lead to mutations突变or carcinogenic致癌
changes.
Phase 1 - Biotransformation
• Includes oxidative, reductive and hydrolytic
reactions.
• In these type of reactions, a polar group is
either introduced or unmasked暴露, so the
xenobiotic becomes more water-soluble and
can be excreted.
• Phase I reactions are non-synthetic in nature &
generally produce a more water soluble & less
active metabolite 代谢物.
• The majority of Phase I metabolites are
generated by a common hydroxylating enzyme
酶 system known as cytochrome P-450.
• Overall reaction is called hydroxylation羟基化
反应
RH + O2 + H2 → ROH + H2O
xenobiotic
(OH) hydroxyl group 羟基
polar and soluble
Figure 3.2 Hydroxylation reaction by cytochrome P-450
xenobiotic
P-450 has a prosthetic group 辅基 inside
The single-electron transfer is achieved by
coupling cytochrome P-450 with another
enzyme called cytochrome P-450 reductase,
which has 2 prosthetic groups.
Both enzymes are embedded inside the cell into
the phospholipid matrix, a component of the
endoplasmic reticulum (ER), especially in the
liver cells.
Phase 2 - Conjugation
• The lipophilic xenobiotic is converted by phase
1 processes into polar, somewhat more
hydrophilic, products may undergo further
transformation into highly-soluble compound.
• These reactions involve covalent attachment of
small polar endogenous molecule such as
glucuronic acid, sulfate, or glycine to form
water-soluble compounds.
• The final compounds have a larger molecular
weight.
Two types of conjugation
1. Electrophilic亲电子 conjugations (the conjugating
agent is an electrophile)
2. Nucleophilic亲核子conjugations (the conjugating
agent is an nucleophile)
3. Some common electrophiles are glucuronide, sulfate,
acetate, glycine, glutamine, etc.
4. Only one nucleophile which is glutathione
Figure 3.10 Mechanism of conjugation of phydroxyacetylalanine with glucuronic acid
Glutathione 谷胱甘肽
Glutathione
•
•
•
•
is a -glutamyl-cysteinyl-glycine tripeptide,
Occurs in most tissues, but especially in the liver.
Involved enzymatic and nonenzymatic reactions.
Nonenzymatically, glutathione captures electrophiles,
acts as a low-molecular-weight scavenger of reactive
electrophilic xenobiotics.
• Enzymatic reaction involving glutathione are catalyzed by
a series of isozymes同工酶commonly known as
glutathione S-transferase (GS).
• Isozymes are enzymes with different chemical
compositions but performing the same catalytic functions
• The substrates include aliphatic and aromatic epoxides,
aromatic and aliphatic halides.
• The conjugated product is further hydrolyzed with the
removal of glutamyl and glycyl residues, followed by Nacetylation by acetyltransferase.
• The end product is mercapturic acid, which is highly
water-soluble and easily excreted in urine.
Reaction between halides and glutathione
Glutathione
• Reactions of organic nitrates 硝酸盐 with glutathione.
• reduction of the organic nitrate to inorganic nitrite
and oxidation of glutathione to its S-S dimer.
• The nitrites formed in such reactions may interact with
amines and thus lead to the formation of carcinogenic
nitroamines 亞硝胺 .
• Another reaction – by glutathione peroxidase  convert
reactive perioxides to alcohols, whereas glutathione is
oxidized.
Reaction between organic nitro
compound and glutathione
Enzyme Induction
• A phenomenon in which a xenobiotic causes  in the
biosynthesis of an enzyme.
• Proceeds via a cytoplasmic receptor-inducer
complex, which in turn interacts with an appropriate
gene to cause an  in production of the enzyme.
• Cytochrome P-450 exists in different forms and these
isozymes are inducible by specific agents.
Cytochrome P-450 Isozymes
• The mixture of isozymes could be separated by gel
electrophoresis into distinct bands, e.g. LM2 and LM4.
• LM2 is inducible by phenobarbital 镇静安眠剂 (PB).
Pretreatment with PB   smooth endoplasmic
reticulum and liver weight, not induce extrahepatic
cytochrome P-450.
• LM4 is inducible by -naphthoflavone and 3methylcholanthrene (3MC). Pretreatment of 3MC  
liver weight, slight  endoplasmic reticulum,  hepatic
and extrahepatic P-450 enzymes.
Inducers of Cytochrome P-450
P-450 inhibitors
• Reversible or irreversible.
• Reversible inhibitors
– Slowly metabolized substrate of P-450
– Occupy the active site of the enzyme and thus retard the
processing of other xenobiotics
– E.g. 2-diethylaminoethyl 2,2-diphenylvalerate
Irreversible P-450 Inhibitors 抗化剂
• An examples is carbon tetrachloride
– Causing peroxidation of lipids
– In turn destroys cell membrane integrity
– Leads to a subsequent loss of P-450
• Assessment of inhibition effect
– Measuring an increase in sleeping time of animals
anesthetized with hexobarbital 环己烯巴比妥
– As hexobarbital is inactivated by cytochrome P-450,
inhibitors of P-450 prolong sleeping time, whereas
inducers shorten it.
Environmental Inducers of P-450
• Insecticide DDT (1,1,1-trichloro-2,3-bis(-chlorophenyl)ethane)
– Induction of P-450
– Reduced the number of mammary tumors produced
by dimethylbenzanthracene, because of the 
production of noncarcinogenic hydroxylation of
dimethylbenzanthracene.
– Induction of epoxide hydrolase, glutathione Stransferase.
Environmental Inducers of P-450
• Other chlorinated hydrocarbon pesticides e.g. aldrin,
dieldrin, hexachlorobenzene and hexachlorohexane.
• Polychlorinated biphenyls (PCBs)
– Used as insulating fluids in capacitors, transformers,
vacuum pumps and gas transmission turbines.
– Induction of -nitrophenol and testosterone glucuronyl
transferases
– Increase liver weight and microsomal protein.
Environmental Inducers of P-450
• TCDD (2,3,7,8-tetrachlorodibenzo--dioxin)
– Very potent inducer
– Formed on incineration of chlorinated organic
substances, found in exhaust and in ash from
municipal incinerators.
– Also formed in the process of pulp bleaching in
paper manufacturing
– A by-product of the manufacturing of a herbicide
and a wood preservative.
Structures of DDT, biphenyl and
tetrachlorodibenzo--dioxin 二恶英
Inducers for phase 2 metabolizing
enzymes
• Found in cruciferous vegetables e.g. broccoli
绿花椰菜, cauliflower 花椰菜, other cabbagerelated plant 甘蓝; 卷心菜.
• Induce glutathione S-transferases and quinone
reductase
• A diet rich in green and yellow vegetables
lowers the risk of cancer in human
Activation of precarcinogens前致癌物
• In some cases the metabolism of xenobiotics
leads to the formation of unstable
intermediates that react with cellular
macromolecules.
• The reaction leads to mutagenic or
carcinogenic transformation.
Precarcinogens
• 2-Naphthylamine萘
– Used in dye染料 manufacturing
– Produce bladder cancer among workers
– Becomes a carcinogen upon N-hydroxylation by
cytochrome P-450
– The Hydroxylamine is stabilized by glucuronidation, and
becomes harmless
– Hydrolyzed back to carcinogenic hydroxylamine by glucuronidase in the kidney or by acidic pH in the urine.
Carcinogenic activation of 2naphthylamine
carcinogen
carcinogen
harmless
Precarcinogens
• Dichloroethane
– Waste product of vinyl chloride production and also a
laboratory solvent
– Its analog, dichloromethane, is used as a gasoline additive
and as an insecticide
– Carcinogen and mutagens
– Conjugate with glutathione to form haloethyl-Sglutathione which is structurally similar to sulfur mustard,
used as war gas during world war I
– Haloethyl-S-glutathione spontaneously forms an unstable
three-member ring that, upon ring opening, reacts with
cellular macromolecules.
Carcinogenic activation of 1.2dichloroethane
Precarcinogens
• Vinyl chloride
– Starting materials in the manufacture of poly(vinyl
chloride) plastics
– Associated with high frequency of angiosarcoma,
a rare liver cancer.
– Carcinogenic activation Involves epoxide
formation
Carcinogenic activation and further
metabolism of vinyl chloride
Precarcinogens
• Aflatoxins 黄曲霉毒素
– Produced by a mold, Aspergillus flavus
– Contaminates crops such as corn and peanuts
– Aflatoxin B1 : can be metabolized by cytochrome
P-450 to a carcinogen
Carcinogenic activation of
aflatoxin B1
Precarcinogens
• Benzo[]pyrene
– A major polycyclic hydrocarbon carcinogen in the
environment
– Occurs in industrial smoke, cigarette smoke and
tar, and in fried, broiled, or smoked food.
– Metabolized by P-450 to form an epoxides, 7,8dihydrodiol-9,10-trans-epoxide that is a
carcinogen with half-life of 8 min. It is probably
long enough to react with DNA.
Carcinogenic activation of
benzo[]pyrene
Precarcinogens
• Nitrosamines亚硝胺
– Formed by the reaction of nitrite ions with
secondary and tertiary amines, e.g.
dimethylamine used in rubber, leather and soap
manufacturing
– The formation of carcinogenic nitrosamines can be
prevented by compounds that compete with
secondary and tertiary amines for the nitrite ion,
e.g. primary amines, ascorbic acid and tocopherol.
Carcinogenic activation of
dimethylamine by reaction with nitrite
ions