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Detoxification

Case detail

A 3-year-old girl was brought into the Emergency Room. She was cold and clammy and was breathing rapidly. She was obviously confused and lethargic. Her mother indicated that she had accidentally ingested automobile antifreeze (ethylene glycol) while playing in the garage. Following gastrointestinal lavage and activated charcoal administration, a nasogastric tube for ethanol was administered. How will ethanol help in relieving the symptoms?

A. Conjugate with ethylene glycol to form a soluble compound

B. Induce the alcohol dehydrogenase enzyme

C. Competitively inhibit the metabolism of ethylene glycol

D. Promote the excretion of metabolite of ethylene glycol

E. All of the above.

The correct answer is- C- Competitively inhibits the metabolism of ethylene glycol.

Ethylene glycol is the major ingredient of almost all radiator fluid products. It is used to increase the boiling point and decrease the freezing point of radiator fluid, which circulates through the automotive radiator. Ethylene glycol is added to prevent the radiator from overheating or freezing, depending on the season.

As with ethyl alcohol and methanol, ethylene glycol is metabolized by alcohol dehydrogenase to form glycoaldehyde. Through interaction with aldehyde dehydrogenase, ethylene glycol is then metabolized to glycolic acid (GA). A profound acidosis often ensues and is attributable to the glycolic acid in circulation. The patient may develop hyperventilation that results from acidemia. This glycolate is then transformed into glyoxylic acid. This glycolate is then transformed into glyoxylic acid. At this point, the molecule may be transformed into the highly toxic oxalate.

Reaction:

 Ethylele glycol

Figure- Metabolism of ethylene glycol

With the formation of oxalate crystals in the urine, calcium oxalate crystals form and accumulate in blood and other tissues. The precipitation of calcium oxalate in the renal cortex results in decreased glomerular filtration and renal insufficiency. Calcium is consumed in circulation, and hypocalcemia may occur.

The rate-dependent step of ethylene glycol metabolism is the alcohol dehydrogenase–catalyzed step. Ethyl alcohol binds much more easily to alcohol dehydrogenase than ethylene glycol or methanol does. Because ethanol is the preferential substrate for alcohol dehydrogenase, the presence of ethanol may essentially block metabolism of ethylene glycol.

Treatment of ethylene glycol poisoning

Emergency and Supportive Measures

For patients presenting within 30–60 minutes after ingestion, the stomach is emptied by gastric lavage. Charcoal is not very effective but should be administered if other poisons or drugs have also been ingested.

Specific Treatment

Patients with significant toxicity (manifested by severe metabolic acidosis, altered mental status, and markedly elevated osmolar gap) should undergo hemodialysis as soon as possible to remove the parent compound and the toxic metabolites. Treatment with folic acid, thiamine, and pyridoxine may enhance the breakdown of toxic metabolites.

Ethanol blocks metabolism of the parent compounds by competing for the enzyme alcohol dehydrogenase. The desired serum ethanol concentration is 100 mg/d. Fomepizole (4-methylpyrazole; Antizol), blocks alcohol dehydrogenase and can be used instead of ethanol.

As regards other options

Alcohol does not form any complex with ethylene glycol, it does not induce alcohol dehydrogenase enzyme and even it does not promote the excretion of metabolites of ethylene glycol.

 

 

 

 

 

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Q.1-The major detoxification reactions involved in phase -1 are all except:

A. Oxidation

B. Reduction

C. Hydrolysis

D. Acetylation

E. Epoxidation

Q.2- The hydroxylated products of phase-1 are converted to soluble metabolites by coupling with polar agents in phase -2 of detoxification reactions by a process called Conjugation. Which of the following is not a conjugating agent?

A. Active acetate

B. Active sulfate

C. Active Methionine

D. Active Glucuronate

E. Active Bicarbonate.

Q.3- The biotransformation reactions are not only meant for removal of toxicity of foreign or endogenous compounds, under certain conditions the toxic potential of certain compounds is rather increased. The reactions involving increase in the toxicity of compounds are called “Entoxification reactions”. Which of the following is not an example of Entoxification reaction?

A. Conversion to Methanol to formate

B. Conversion to ethanol to acetate

C. Conversion of procarcinogens to ultimate carcinogens

D. Conversion of p-methyl amino Azo benzene to p-dimethyl amino Azo benzene.

E. Methylation of mercury

Q.4- Acetanilide is a constituent of analgesic drugs. It is detoxified to form p-Acetyl amino phenol. Which of the following processes is involved in this detoxification process?

A. Oxidation

B. Reduction

C. Hydrolysis

D. Acetylation

E. Methylation

Q.5- Digitalis a cardiac glycoside is detoxified forming a sugar and an aglycone by the process of-

A. Oxidation

B. Reduction

C. Hydrolysis

D. Acetylation

E. Methylation

Q.6- Phase -1 reactions are mainly catalyzed by a class of enzymes referred to as-

A. Mono- oxygenases

B. Mixed function oxidases

C. Cytochrome P 450 enzyme system

D. Al of the above.

E. None of the above

Q.7- Which of the following statements is incorrect about conjugation reactions?

A. Conjugation reactions can occur independently

B. The reactions can occur after phase -1 reaction

C. Most of the reactions take place in the liver

D. After conjugation most of the compounds are rendered non toxic

E. The polarity of the xenobiotics is decreased upon conjugation

Q.8- Which of the following compounds is detoxified by Glucuronidation reactions?

A. Aspirin

B. Methanol.

C. Bilirubin

D. Phenyl acetate

E. Bromobenzene

Q.9- Which of the following enzymes is involved in the Glucuronidation reactions?

A. UDP-G Pyro phosphorylase

B .UDP dehydrogenase

C. UDP Glucuronyl transferase

D. Glucan transferase

E. Glucuronidase

Q.10- Sulfation is an important method of detoxification of foreign compounds. Which of the following acts as a source of sulfate?

A. Sulphuric acid

B. Hydrogen sulphide

C. Phospho adenosine phospho sulfate

D. Methionine

E. Cysteine.

Q.11- The compounds conjugated by Methylation are all except:

A. Nicotinamide

B. Isoniazid

C. Oestrogen

D. Norepinephrine

E. Homocysteine

Q.12- Conjugation of foreign compounds with cysteine results in the formation of:

A. Hippuric acid

B. Mercapturic acid

C. Uric acid

D. Sulphuric acid

E. None of the above.

Q.13- The characteristic mousy odor of urine in a patient suffering from phenyl Ketonuria is due to the compound:

A. Phenyl acetyl glutamine

B. Phenyl Glucuronate

C. Phenyl lactate

D. Phenyl pyruvate

E. Phenyl Mercapturic acid

Q.14- The enzyme catalyzing the conjugation of glutathione with xenobiotics is:

A. Glutathione S-transferase

B. Glutathione peroxidase

C. Glutathione reductase

D. Glutathione synthase

E.  Glutathione Oxidase

Q.15- Which of the following reactions is used as an index for assessing the liver functions?

A. Formation of Hippuric acid

B. Methylation reactions

C. Acetylation reactions

D. Detoxification of ethanol

E. Glucuronidation reactions

Q.16- Which of the following is incorrect about Cytochrome P 450 enzyme system?

A. Most of these enzymes are inducible in nature

B. Some exhibit genetic polymorphisms

C. All of them are hemo proteins

D. They can be inhibited by drugs

E. They are Dioxygenases in their mechanism of action

Q.17- UDP-Glucuronic acid, the glucuronyl donor, in the Glucuronidation reactions is formed in the pathway of…

A. Glycolysis

B. Hexose mono phosphate

C. Uronic acid

D. Gluconeogenesis

E. Glycogenolysis

Q.18- Aspirin has antipyretic, analgesic and anti-atherogenic properties. It is detoxified by:

A. Oxidation

B. Reduction

C. Hydrolysis

D. Methylation

E. Glucuronidation

Q.19- The role of glutathione in tissues includes all except:

A. Participates in the decomposition of hydrogen peroxide

B. Biologically active in the oxidized form

C. Participates in the absorption of amino acids

D. Participates in the detoxification reactions

E. Participates in the activation of Methionine

Q.20- A 3 year old girl was brought into the Emergency Room. She was cold and clammy and was breathing rapidly. She was obviously confused and lethargic. Her mother indicated that she had accidentally ingested automobile antifreeze while playing in the garage. Following gastrointestinal lavage and activated charcoal administration, a nasogastric tube for ethanol was administered. How will ethanol help in relieving the symptoms?

A. Conjugate with methanol to form a soluble compound

B. Induce the alcohol dehydrogenase enzyme

C. Competitively inhibit the metabolism of methanol

D. Promote the excretion of metabolite of methanol

E. All of the above.

 

Key to answers

1)-D, 2)-E, 3)-B, 4)-A, 5)-C,6)-D,7)-E,8)-C,9)-C,10)-C,11)-B,12)-B,13)-A,14)-A,15)-A,16)-E,17)-C,18)-C,19)-B,20)-C.

 

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Q. 1- What is the normal physiological concentration of Hydrogen ion in body fluids?

A) 40 nEq/L

B) 24 mEq/L

C) 400 mEq/L

D) 7.4 nEq/L

E) 100 mEq/L

Q.2- Which of the following is not a source of hydrogen ion in the body?

A) Ingestion of Citrus fruits

B) High protein diet

C) Ingestion of red meat

D) Starvation

E) Chronic alcohol consumption

Q.3- Which of the following is the most important chemical buffer of the plasma?

A) HCO3 /H2 CO3

B) HPO42―/H2PO4

C) Organic Phosphate Esters

D) Proteins

E) Hemoglobin

Q.4- A primigravida in labor is breathing rapidly, what you expect out of the following

A) Metabolic Acidosis

B) Metabolic Alkalosis

C) Respiratory Acidosis

D) Respiratory Alkalosis

E) Any of the above.

Q.5- The Henderson-Hasselbalch equation is represented as-

A) pH = pK + log (A/HA)

B) pH = pK + log (HA/A)

C) pH = pK – log(A/HA)

D) pH = pK – log(HA/A)

E) pH = pK + log(H+/HA)

Q.6- Buffering effect of a buffering solution is optimum at :

A) pH ranges close to pKa± 2 pH units

B) pH = pKa ±3 pH units

C) pH = pKa ±5 pH units

D) pH = pKa

E) None of the above.

Q.7- The pH of extracellular fluid must be maintained between:

A) 6 to 7.4

B) 7 to 7.2

C) 7.35 to 7.45

D) 7.5 to 8

E) 8 to 8.5

Q.8- All are true for renal handling of acids in metabolic acidosis except

A) Hydrogen ion secretion is increased

B) Bicarbonate reabsorption is decreased

C) Urinary acidity is increased

D) Urinary ammonia is increased

E) Renal glutaminase activity is increased

Q.9- Which of the following is most appropriate for a female suffering from Insulin dependent diabetes mellitus with a pH of 7.2, HCO3-17 mmol/L and pCO2-20 mm Hg?

A) High anion gap metabolic Acidosis

B) Metabolic Alkalosis

C) Respiratory Acidosis

D) Respiratory Alkalosis

E) Normal anion gap metabolic acidosis

Q.10-A 50-year-old homeless man was brought to the emergency room in a stuporous state. Below are his lab results, Bicarbonate 10mEq/L (24-26), pH 7.2 (7.35-7.45), PCO2 25mmHg (35-45), Alcohol 40mmol/L (0), Osmolality 370mOsm/L (280-295), Glucose 50mg/dl (60-110) BUN 40mg/dl (5-22). What is the acid-base status?

A) Metabolic acidosis and metabolic alkalosis

B) Metabolic acidosis with partial respiratory compensation

C) Respiratory acidosis and partial metabolic compensation

D) Respiratory acidosis

E) Metabolic alkalosis

Q.11- A 44-year-old man is brought to the emergency room stuporous and obtunded. Serum chemistries are: HCO3 = 42 mEq/L; arterial pH = 7.5; PCO2 = 50mmHg. What is the acid-base status?

A) Metabolic acidosis and metabolic alkalosis

B) Metabolic acidosis with partial respiratory compensation

C) Respiratory acidosis and partial metabolic compensation

D) Respiratory acidosis

E) Metabolic alkalosis

Q.12-The medical student next to you, realizing that there is an examination question on acid base balance, begins nervously hyperventilating and then faints. You make him breathe into a paper bag and he recovers. If you had drawn and analyzed his blood when he fainted you would have expected to see :

A) Decreased pH, decreased pCO2

B) Decreased pH, elevated pCO2

C) Elevated pH, decreased pCO2

D) Elevated pH, elevated pCO2

E) Normal pH, normal pCO2

Q.13- All except one are examples of entoxification:

A) Conversion of methanol to formaldehyde

B) p- methyl amino benzene to p-dimethyl amino azo benzene

C) Conversion of procarcinogens to Ultimate carcinogens

D) Conversion of Aspirin to Acetic acid and Salicylic acid

E) Conversion of Ethyl alcohol to Acetaldehyde.

Q.14- In physiological jaundice of new-born, due to less availability of substrate and immature enzyme system, there is an impaired formation of soluble, non toxic form of bilirubin which is :

A) Bilirubin Sulphate

B) Bilirubin Phosphate

C) Bilirubin diglucuronate

D) Bilirubin Acetate

E) Methylated Bilirubin

15) In  phenylketonuria (a congenital disorder of phenylalanine metabolism that occurs due to deficiency of phenylalanine hydroxylase), there is impaired conversion of phenylalanine to tyrosine. The excess phenylalanine is detoxified and excreted in urine. Which of the following conjugating agents is used for detoxification of phenylalanine?

A) Glutathione

B) Glutamine

C) S-Adenosyl Methionine

D) Active Sulfate (PAPS)

E) D- Glucuronic acid

16) Which of the following is not a cause of secondary dehydration?

A) Excessive sweating

B) Comatose patient

C) Vomiting

D) Diarrhea

E) Congestive heart failure

17) The urinary concentration of sodium chloride (NaCl) ranges between:

A) 2-6 G/litre

B) 4-8 G/litre

C) 5-10 G/litre

D) 6-16 G/litre

E) None of the above

18) The minimum excretory volume to eliminate waste products from the body in dehydration is :

A) 100-200ml

B) 200-400 ml

C) 500-600 ml

D) 1500 ml

E) 600-800 ml

19) Aldosterone acts by promoting:

A) Excretion of Potassium

B) Reabsorption of potassium

C) Reabsorption of sodium

D) Excretion of sodium

E) Reabsorption of sodium and excretion of Potassium

20) Which of the following is not a cause of hypokalemia?

A) Renal tubular acidosis

B) Cushing syndrome

C) GI losses

D) Crush injuries

E) Insulin administration

Key to answers

1)- A, 2)- A, 3)-A, 4)-D, 5)-A, 6)-A, 7)-C, 8)-B, 9)-A, 10)- B, 11)-E, 12)-C, 13)-D, 14)-C, 15)-B, 16)-B, 17)-D, 18)-C, 19)-E, 20)-D.

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Q. 1-Outline the process of detoxification of following compounds

i) Aspirin            ii) Digitalis            iii)  Polycyclic aromatic hydrocarbons

Q.2- “The complete detoxification of xenobiotics requires the concerted action of enzymes in Phase 1 and Phase 2 reactions, justify the statement giving at least one example

Q.3- Discuss in detail about the detoxification of Bilirubin and elaborate the clinical significance associated with these reactions.

Q.4-A 4- year-old – child accidently ingested Methanol and was rushed to hospital.  He was given Ethanol as an antidote and was kept under observation. What is the basis of using ethanol? Explain the steps of detoxification by giving the relevant reactions.

Q.5- Give a brief account of cytochrome P 450 enzyme system. Discuss the important characteristics.

Q.6-What is meant by entoxification of xenobiotics? Give examples in support of your answer.

Q.7- Give a detailed account of phase 1 reactions of detoxification, suggest examples for each set of reactions.

Q.8- Discuss the role of glycine as a conjugating agent. What is the significance of this reaction?

Q.9- Give an account of transmethylation reactions undertaken for the detoxification of xenobiotics.

Q.10-Give a brief description of the role of amino acids in the detoxification of foreign compounds, discuss the significance of each reaction.

Q.11- Give a detailed account of phase II reactions of detoxification. Give examples for each category of reactions.

Q.12-  What  are slow and fast acetylators ? Explain in relation to acetylation of Isoniazid.

Q.13- What are the effects of xenobiotics? Discuss the role of glutathione as a defense peptide from the effect of peptides.

Q.15- Discuss the role of BAL in the detoxification of toxic metals.

Q.16- Outline the step of detoxification of cyanide, what are the clinical implications of cyanide toxicity?

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This is an enzyme of Uronic acid pathway. Uronic acid pathway is an alternative pathway for the oxidation of glucose that does not provide a means of producing ATP, but is utilized for the generation of the activated form of glucuronate, UDP-glucuronate which is mainly used for detoxification of foreign chemicals, some endogenous compounds like bilirubin,certain hormones, metabolites and for the synthesis of mucopolysaccharides.This pathway also produces Ascorbic acid in certain animals.

The unutilized Glucuronate produced in this pathway is converted to Xylulose-5 P which is further metabolized through HMP pathway.(Figure-1)

 

 

Figure1-showing the overview of Uronic acid pathway

 

Biological significance of UDP –Glucuronyl Transferase

UDP glucuronate the active form of Glucuronic acid, can readily donate the Glucuronic acid component under the catalytic activity of UDP –Glucuronyl Transferase for the following functions-

1)   Detoxification of foreign compounds and drugs- During detoxification, the glucuronate residues are covalently attached to lipid soluble substances. Since glucuronate residues are strongly polar, their attachment imparts polar character to these substances, making them water-soluble and readily excretable. Bilirubin, certain hormones and drugs are made more polar for renal excretion in this manner. UDP-Glucuronic acid is the Glucuronyl donor, and a variety of glucuronosyl transferases, present in both the endoplasmic reticulum and cytosol, are the catalysts. Molecules such as 2-acetylaminofluorene (a carcinogen), aniline,benzoic acid, meprobamate (a tranquilizer), phenol, and many steroids are excreted as glucuronides. The glucuronide may be attached to oxygen, nitrogen,or sulfur groups of the substrates. Glucuronidation is probably the most frequent conjugation reaction.

2)   Synthesis of Mucopolysaccharides-UDP Glucuronic acid is an essential component of Hyaluronic acid and heparin.

3)   Conjugation of Bilirubin-Bilirubin is nonpolar and would persist in cells (eg, bound to lipids) if not rendered water-soluble. Hepatocytes convert bilirubin to a polar form, which is readily excreted in the bile, by adding Glucuronic acid molecules to it. This process is called conjugation.

The conjugation of bilirubin is catalyzed by a specific Glucuronyl transferase.The enzyme is mainly located in the endoplasmic reticulum, uses UDP-Glucuronic acid as the glucuronosyl donor, and is referred to as bilirubin-UGT. BilirubinMonoglucuronide is an intermediate and is subsequently converted to the diglucuronide. Most of the bilirubin excreted in the bile of mammals is in the form of bilirubin diglucuronide.(Figure-2)

 

 

Figure- 2-showing the conjugation of bilirubin

Clinical significance Diminished activity of Bilirubin UDP Glucuronyl Transferase (UGT)

1) Neonatal “Physiologic Jaundice”

This transient condition is the most common cause of Unconjugated hyper-bilirubinemia. It results from an accelerated hemolysis around the time of birth and an immature hepatic system for the uptake, conjugation, and secretion of bilirubin. Not only is the bilirubin-UGT activity reduced, but there probably is reduced synthesis of the substrate for that enzyme,UDP-glucuronic acid. Since the increased amount of bilirubin is unconjugated,it is capable of penetrating the blood-brain barrier when its concentration in plasma exceeds that which can be tightly bound by albumin (20–25 mg/dL). This can result in a hyperbilirubinemic toxic encephalopathy, or kernicterus,which can cause mental retardation. Because of the recognized inducibility of this bilirubin UGT enzyme system, phenobarbital has been administered to jaundiced neonates and is effective in this disorder. In addition, exposure to blue light (phototherapy) promotes the hepatic excretion of unconjugated bilirubin by converting some of the bilirubin to other derivatives such as maleimide fragments and geometric isomers that are excreted in the bile.

2) Crigler-Najjar Syndrome, Type I; Congenital Nonhemolytic Jaundice

a) TypeI Crigler-Najjar syndrome -is a rare autosomal recessive disorder. It is characterized by severe congenital jaundice(serum bilirubin usually exceeds 20 mg/dL) due to mutations in the gene encoding bilirubin-UGT activity in hepatic tissues. The disease is often fatal within the first 15 months of life. Children with this condition have been treated with phototherapy, resulting in some reduction in plasma bilirubinlevels. Phenobarbital has no effect on the formation of bilirubin glucuronides in patients with type I Crigler-Najjar syndrome. A liver transplant may be curative. It should be noted that the gene encoding human bilirubin-UGT is part of a large UGT gene complex situated on chromosome 2. Many different substrates are subjected to glucuronosylation, so many glucuronosyl transferases are required.

b) Crigler-Najjar Syndrome Type II-This rare inherited disorder also results from mutations in the gene encoding bilirubin-UGT, but some activity of the enzyme is retained and the condition has a more benign course than type I. Serum bilirubin concentrations usually do not exceed 20 mg/dL. Patients with this condition can respond to treatment with large doses of phenobarbital.

3) Gilbert Syndrome

Again,this relatively prevalent condition is caused by mutations in the gene encoding bilirubin-UGT. It is more common among males. Approximately 30% of the enzyme’s activity is preserved and the condition is entirely harmless.

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