Biochemistry, Detoxification

1. VETERINARY BIOCHEMISTRY
Detoxification:
Metabolism of Xenobiotic

2. Detoxification of Xenobiotics
 Xenobiotics – no nutritive value, no use for the
body, Can be harmful, eg. Drugs, Chemicals,
food contaminants, food additives
 Mostly lipophilic (hydrophobics) and made
hydrophilic for excretion
 Liver is the principal organ
 Other organs: SI, Kidney

3. Detoxification: Metabolism of Xenobiotic
 Continuous exposure of animal to foreign compounds (drugs,
pollutants, food additives, pesticides etc)
 Certain compounds produced in large intestine eg. Indol,
cadaverine, tyramine, phenol, Bilirubin
 Detoxification is converting foreign substance in more soluble and
easily excretable form. It is also called biotransformation or
metabolism of xenobiotics.
 Knowledge of metabolism of detoxification is necessary for
understanding toxicology and pharmacology
 Site of detoxification- Liver , Kidney, intestine

4. Detoxification of Xenobiotics
 Metabolism involves
Phase-I Reaction & Phase –II reactions
1. Phase-I
Cyp450
RH R-OH (Primary metabolite)
Reduction
Oxidation
Hydrolysis
hydroxylation

5. Detoxification of Xenobiotics
 2. Phase-II
Molecule is made more water soluble
 Conjugation
 Sulfation
 Methylation
 Glucuronidation
 Secondary metabolite is suitable for
excretion via kidney, intestine, sweat gland

6. Detoxification of Xenobiotics
 Most common Phase-I reactions are catalyzed
by Cyp450 enzymes
 Different enzymes involved in Biotransformation
1. Cyp450 monooxygenase
2. Glutathione s transferase
3. Sulfotransferase
4. UDP-Glucoronosyl transferase
5. Methyl Transferase
6. Acetyl Transferase
7. Amidase-esterase

7. Detoxification (Phase-I)
1. Oxidation: alcohol, aldehyde, amines, aromatic hydrocarbons, sulfur
compounds detoxified by oxidation.
 Introduction of –OH group into an aromatic ring increase water
solubility.
 Most of the oxidation reactions are catalyzed by monooxygenase or
cytochrome P450.
1. Alcohol : Methanol formic acid
Ethanol Acetic acid
Benzyl alcohol Benzoic acid
2.Aldehyde: Benzaldehyde Benzoic acid
3.Aliphatic amine Aliphatic acid+ urea
4.Aromatic hydrocarbon: Benzene hydroxy phenol
5.Sulfur compound oxidized to sulfuric acid

8. Detoxification (Phase-I)
 2.Reduction:
1. Picric acid Picramic acid
2. Chloral Trichloroethanol
3. Nitrobenzene Aminobenzene
 3.Hydrolysis
1. Aspirin (Acetylsalicylic acid) salicylic acid+
acetic acid
2. Atropine Tropic acid + Tropine
1. Procaine p-Aminobenzoic acid
+Diethylaminoethanol

9. Detoxification(Phase-II)
 Conjugation: It is a process by which foreign compounds combines
with the substances produced in the body and non polar compound
becomes polar
 Glucuronic acid, glycine, cysteine, glutamine, methyl group,
sulphate, acetic acid and thiosulphate are most common conjugating
substances.
 The active form of glucuronic acid is UDP-glucuronic acid.
 The microsomal enzyme UDP- glucuronyle transferase participate in
glucuronide formation.
 The compound containing –OH group are converted to glucuronide
by the reaction with UDP-glucuronic acid.
Phenol+ UDP-glucuronic acid Phenyl glucuronide + UDP

10. Detoxification(Phase-II)
 Glucuronic acid conjugation:
1. Phenol Phenyl glucuronide
2. Benzoic acid Benzoyl glucuronide
3. Billirubin+ UDP-Glucuronic acid Bilirubin glucuronide
4. Many steroids metabolites are excreted as glucuronides.
 Glycine:
Hippuric acid is form when glycine is conjugated with benzylcoA.
Phenylacetic acid+ glycine phenylaceturic acid
Cholic acid+glycine Glycocholic acid.
 Glutathione: (Glu-Cys-Gly):
Wide variety of organic compounds get conjugated with cysteine of
glutathione.

11. Cyp450 enzymes
Cyp3A4
 Cyp- superfamily
 3 – subfamily
 A – Substrate
 4 - Isoform

13. Cyp450 enzymes
 Flavin and heme containing encoded by 57 functional
genes
 Spectral absorption 450
 Use NADPH+ H as reducing molecule to reduce O2
Cyp450
 RH R-OH
H2O
 They all causes oxidative, peroxidative, reductive
degradation of exogenous and endogenous chemicals
 Found in sER and microsomes

14. Cyp450 enzymes
 Cyp3A4 isoform accounts for 30-40% cyp450 enzyme in
liver and 70% in SI
 Cyp3A4 metabolizes greatest number of drugs
 Drug metabolism by Cyp3A4 is based on the affinity of the
enzyme to the specific drug thereby inducing competition
amongst the drugs for the metabolism
 Many substances or drugs can impair the metabolizing
ability of Cyp3A4. e.g. grape fruit juice inhibitory effect on
cyp3A4 for metabolism of HMGCoA inhibitory drug statin

15. Alcohol metaboilism
 Ethanol detoxified in liver either by
1. Cyp2E1
2. ADH (Alcohol Dehydrogenase)
3. Catalase
Acetaldehyde

16.  Cyp2D6 is important in metabolism of
codeine and it is converted to Morphine
 Cyp2D6 polymorphism
 It metabolizes the drug slower or ultra-rapid

17. Cyp450 enzymes
 Cyp2E1 is alcohol metabolizing Cyp450
enzyme
 Microsomal Ethanol oxidizing system(MEO)
 Cyp450 enzymes generates free radicals
(ROS) as an intermediate in the reaction

18. Biotransformation of Xenobiotics
 Vinyl chloride Chloroethylene oxide
 Phase I Phase-II
 Cyp2E1
 React with G of DNA Chloroacetaldehyde
 G-s transferase
 Angiosarcoma of liver
 Conjugate with Glutathione
 Excreted

20. Biotransformation of Xenobiotics
 Vinyl chloride is readily and rapidly absorbed via
inhalation, ingestion and through the skin
 At room temperature vinyl chloride is a gas, so inhalation
is the major route exposure
 Following absorption, it is distributed through the body,
with the highest concentrations found in the liver and
kidneys, followed by the lungs and spleen
 Vinyl chloride is mainly metabolized in the liver into
reactive metabolites
 Vinyl chloride is mainly excreted in the urine as
thiodiglycolic acid

21. Biotransformation of Xenobiotics
 Repeated exposure to vinyl chloride may cause
liver toxicity, neurological and behavioral
symptoms
 Vinyl chloride is considered to be carcinogenic

22. Biotransformation of Xenobiotics
Aflatoxin B-1
 Produced by fungus Aspergillus flavis
 Cyp2A1 bio-transform Aflatoxin B-1 into 8,9
epoxide form which is more toxic form
 Toxification of less toxic to more toxic compound
 It adduct with Guanine and introduce mutation
leading to carcinogenic effect
 It can also hepatotoxic

23. Cyp450 transformation reaction
Acetaminophen or Paracetamol (Tylenol)
 Metabolized in liver by sulfation, glucuronidation by
Cyp2E1
 Safe Excretion by Kidney in urine
 When Acetaminophen is taken in correct therapeutic dose
only < 10% of NAPQI is formed which is handled by GST
by using GSH
 When the dosage exceed therapeutic range, sulfation and
glucuronidation overwhelm
 More than 10% NAPQI formed via Cyp2E1

25. Biotransformation of Xenobiotics
 Limited level of GSH in the liver may not be able to
metabolized it thus leading to hepatocyte death
 Cyp2E1 is induced by alcohol, thus alcohol abuse
will have increased sensitivity to acetaminophen /
paracetamol toxicity
 N-acetyl cysteine is an antidote for acetaminophen
/ paracetamol toxicity

27. Biotransformation of Xenobiotics