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X Understanding Disease > Xenobiotics > Flashcards

Xenobiotics Flashcards

1
Q

what is a xenobiotic?

A

Foreign substance with no biological benefit nutritionally

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2
Q

what does Biotransformation mean?

A

xenobiotic metablolism

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3
Q

what does detoxification mean?

A

inappropriate as purpose of metabolism is to ensure excretion and changes in toxicity coincidental

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4
Q

what 5 classes of xenobiotics are there?

A 
Food additives and anutrients
Drugs
Pesticides and herbicides
Industrial chemicals
Bacterial metabolites
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5
Q

what is an example of Food additives and anutrients xenobiotics?

A

Colourings, pigments

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6
Q

what example of drug xenobiotics are there?

A

Aspirin, oral contraceptives

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7
Q

what example of pesticides and herbicides xenobiotics are there?

A

Fly sprays, weed killers

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8
Q

what examples of industrial chemical xenobiotics are there?

A

Dyes, detergents, bleaching agents

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9
Q

what examples of bacterial metabolite xenobiotics are there?

A

Putrescine, other amines

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10
Q

what means of exposure to xenobiotics are there?

A

accidental
coincidental
deliberate

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11
Q

what is an example of accidental exposure?

A

inhalation of garden sprays, oven cleaner, furniture polish etc

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12
Q

what is an example of coincidental exposure?

A

occupational exposure, inhalation of atmospheric pollutants

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13
Q

what is an example of deliberate exposure?

A

applications of cosmetics/deodorant, administration of a drug

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14
Q

what is the mechanism of metabolism and excretion of xenobiotics?

A

1) administration via gut, Lungs, Skin, Parenteral routes
2) direct to tissue retention/metabolism to tissue retention
3) direct to excretion
4) metabolism to excretion
5) excretion can return xenobiotic to Enterohepatic circulation, to return to gut, Lungs, Skin, Parenteral routes

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15
Q

what are the routes of excretion of xenobiotics?

A

urine, bile/faeces, expired air, swear, tears, breast milk

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16
Q

what is the influence of the physical properties of the xenobiotics on excretion?

A

if hydrophillic water soluble: excreted unchanced

if hydrophobic lipid soluble: retained in membrane unchanged

somewhere on spectrum between hydrophillic/hydrophobic: metabolised to increase water solubility to excrete

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17
Q

what happens if the xenobiotic is similar to a natural metabolite?

A
  1. Bind to specific transport/binding proteins in cells/circulation
  2. Enter cells/organelles via specific transporters
  3. Be metabolised by specific enzymes e.g. fluoroacetate -> fluorocitrate
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18
Q

what is the aim of metabolism?

A

increase water solubility for excretion

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19
Q

what is the mechanism of phase 1 metabolism?

A
  • Prepare molecule for phase 2
  • insertion of a reactive group via oxidation, reduction or hydrolysis
  • Usually little change in H2O solubility
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20
Q

what is the mechanism of phase 2 metabolism?

A
  • Attach conjugating group (conjugation) e.g. sulphate
  • Requires energy
  • Usually increases H2O solubility
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21
Q

what 3 enzymes catalyse oxidation?

A

Cytochrome P450 family
Flavin-containing monooxygenases
Prostaglandin synthetase

22
Q

what does the Cytochrome P450 family enzyme do in oxidation?

A

Catalyse insertion of oxygen atom
XH -> XOH
“activate” xenobiotics

23
Q

what does the Flavin-containing monooxygenases enzyme do in oxidation?

A

Mechanism of action similar to P450 family

Fewer enzymes

24
Q

what does the Prostaglandin synthetase enzyme do in oxidation?

A

Catalyses prostaglandin formation

Required electrons derived from various substances including some xenobiotics

25
Q

what is an example of a reduction reaction?

A

protonsil -> sulphanilamide

26
Q

which groups do reduction reactions usually involve? ?

A

nitrogen-containing groups

27
Q

what is a typical example of a hydrolysis reaction?

A

Acetylsalicylic acid (aspirin) + H2O
->
Acetic acid + Salicylic acid

28
Q

what 4 types of conjugating groups/reactions are there?

A
  1. Glucuronic acid/glucuronidation
  2. Sulphate/sulphation
  3. Methyl/methylation
  4. Amino acids
29
Q

which conjugating group/reaction reduces H2O solubility? when?

A

Methyl/methylation

when too much paracetamol

30
Q

how is conjugation used to eliminate bilirubin?

A

(Spleen)
Haem catabolism -> Bilirubin

(Liver)
Bilirubin conjugated to Bilirubin diglucuronide

Excretion into bile

31
Q

how is conjugation used to produce bile salts?

A

(Liver)
Cholesterol -> Cholyl CoA

Conjugated to Glycocholate/taurocholate

32
Q

what factors affect metabolism?

A 
Genetic factors
Tissue
Sex
Stage of development
Age
Induction
Interaction with other xenobiotics, e.g. drugs
33
Q

what is the responsiveness to drugs affected by?

A

genetics - activity of metabolising enzymes

34
Q

how does the pharmacodynamics of CYP2C9 affect drug responsiveness?

A

CYP2C9 (form of P450) (metabolises warfarin) –

individuals with at least 1 copy of allele 2 require 17% less
those with at least 1 copy of allele 3 require 37% less, than those homozygous for allele 1

Titrate dose according to enzyme activity

35
Q

why can xenobiotics cause problems in one tissue but not another?

A
  1. Selective accumulation
  2. Different route for metabolism
  3. Potential for further metabolism
36
Q

what is an example of a xenobiotic causing problems in one tissue but not another?

A 
(Liver)	2-Naphthylamine
			Oxidation 	
		N-hydroxy-2-naphthylamine*
			Conjugation
		Glucuronide
(Bladder) 	Glucuronide 
		hydrolysed due to low urinary pH
	N-hydroxy-2-naphthylamine*
	*Mutagenic
37
Q

why is it harmful in the bladder but not the liver?

A

liver conjugates it but bladder doesn’t. it builds up in the bladder

38
Q

what happens when there is no metabolism of the xenobiotic?

A

accumulation

39
Q

what happens when there is partial metabolism of xenobiotic?

A

accumulation of phase 1 product

40
Q

what happens if a xenobiotic is metabolised via an alternative route? give an example

A

Can result in selective toxicity and/or mutagenicity if substance that accumulates is harmful

e.g. Phenytoin (anti-convulsant) metabolised by prostaglandin synthetase = mutagen in foetal tissues

41
Q

what affect do xenobiotics often have on xenobiotic metabolising enzymes?

A

often increase activity

42
Q

what non-specific xenobiotic metabolising enzymes are there?

A

phenobarbital

43
Q

what specific xenobiotic metabolising enzymes are there?

A

phenylbutazone

44
Q

does interactions does CYP2D6 have with other xenobiotics?

A

inhibited by fluoxetine -> reduced metabolism of all other drugs metabolised by this enzyme

45
Q

does interactions do Furanocoumarins have with other xenobiotics?

A

occur specifically in grapefruit

inhibit intestinal CYP3A4 -> increased bioavailability of a number of drugs including some statins

46
Q

what is Phenylbutazone?

A

an anti-inflammatory agent

Induces enzymes for which it is substrate
Enzymes also act on warfarin (anti-coagulant)

47
Q

what happens if Phenylbutazone and warfarin are both given together?

A

must increase dose of warfarin

if phenylbutazone withdrawn, must decrease dose of warfarin as induction effect lost

48
Q

what does activation mean?

A

Enhancement of toxicity &/or mutagenicity as a result of attempts to carry out biotransformation

49
Q

what are the causes of activation?

A

Metabolism more reactive product

and/or

Accumulation of reactive metabolite because unable to be metabolised

50
Q

what are the key features of activation?

A 
May result from any type of reaction
Some enzymes/processes more likely to cause activation especially:
	Certain forms of cytochrome P450
	Prostaglandin synthetase
	Reduction

X Understanding Disease (10 decks)

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