Theme 2: Drug metabolism: individual enzymes (L7-12) Flashcards
1
Q
What are the different sites of metabolism?
A
Liver (largest)
GI
Kidney
Skin
Lung
Plasma (hydrolysis)
Brain
2
Q
What are the different functions of the liver?
A
Detoxification
Metabolism of carbs, lipids and proteins
Synthesis of plasma proteins
Storage of glycogen, vitamins and minerals
Bile production
3
Q
What is the structure of the liver?
A
Contains liver lobules made up of hepatomata’s mainly with the hepatic artery and portal vein
A bile ductule and central vein
4
Q
What are the different cell types in the liver?
A
Hepatocytes (70%)
Cholangiocytes (bile)
Kupffer cells
Stellate cells (vitamin A)
Endothelial cells (structure)
Fibroblasts (structure)
5
Q
What are the cells that allow liver regeneration?
A
Hepatocyte proliferation
Liver stem/progenitor cells into intracellular
6
Q
Is liver regeneration continuous?
A
NO - repeated damage and exhaustion leads to fibrosis and cirrhosis
7
Q
What is the function of microsomes enzymes?
A
They are contained in the smooth ER used for drug metabolism
8
Q
Why is the smooth ER important in hepatocytes?
A
As it is a major site of drug metabolising enzymes
Microsomal fraction - pinched off ER vesicles
Different drugs can increase ER content
9
Q
How are microsomes isolated?
A
Cells disrupted
Homogenisation
Centrifugation (3rd pellet)
10
Q
What are the phase I catabolic reactions for drug metabolism?
A
Functionalisation reactions, functional group added produces more polar and excretable molecule
Phases can go in alternative orders
11
Q
What are properties of cytochrome P450s?
A
57 human CYPs
Overlap substrates
Contain genetic polymorphisms
Genotype-phenotype relationship
Induced and inhibited
Drug-drug interactions
12
Q
What is one of the most used CYP?
A
CYP3A4 - wide substrate specificity
13
Q
How can CYP3A4 be induced and inhibited by other factors?
A
St John’s Wort - induction of CYP3A4 and P-gp
Grapefruit juice - inhibition
14
Q
What are the phase II anabolic reactions?
A
Conjugation reactions detoxifying agents and prepare for excretion
15
Q
What are the phase II enzymes?
A
UDP-glucuronosyltransferases (UGTs)
Sulfotransferases (SULTs)
Glutathione S-transferases (GSTs)
N-acetyltransferases (NATs)
Methyltransferases
Amino acid conjugating enzymes (e.g. glycine and glutamic acid)
16
Q
Which factors can affect first pass metabolism?
A
Genetic variations between individuals in liver and GI
Variations between blood flow liver and GI too
Gut microbiota
17
Q
Why are prodrugs so advantageous?
A
Increase solubility
Higher 1st pass
More stable
Absorbed well
Target specific
Taste alterations for children
Reduce pain at site of administration
18
Q
What are examples of prodrugs?
A
Heroin deacetylated to morphine
Codeine demethylated using CYP2D6 to morphine
Cyclophosphamide (CPA) hydroxylated to 4-OH-CPA (for chemo)
19
Q
What are the cytochrome P450 (CYP450) enzymes?
A
Most important family in phase I metabolism of xenobiotics and endobiotics
ER and mitochondria of eukaryotes
Single haem as prosthetic group
Complex shows max absorbable at 450nm when reduced and CO added
20
Q
What is the timeline of the CYP450 enzymes?
A
<1960s: known liver ER metabolised using NADPH and O2
60-80s: Spectroscopy identification - reduced and CO present at 450nm
Multiple forms of enzyme and reconstitution experiments
>1980s: Protein purification and cloning genes
50 different forms known
>2000s: X-ray structures
21
Q
How are CYP450s named?
A
Root CYP followed by family (1,2,3,etc.), subfamily and form number
22
Q
What is the percentage homology between families?
A
40%
23
Q
What is the sequence similarity between CYP450 subfamilies?
A
Between 40 and 70%
24
Q
How far back can CYP450s be traced to?
A
1.5billion years ago
25
What are the different functions of CYP450s?
Xenobiotic metabolism
Steroid, fatty acid and vitamin oxidation
Steroid biosynthesis
26
Which CYP450s are mainly involved in xenobiotic metabolism?
Families 1,2 and 3
27
Which CYP450s are used in steroid, fatty acid and vitamin oxidation?
Variety of isoforms (xenobiotic metabolism and specialisations)
e.g. CYP4 FA oxidation and CYP26 retinoic acid oxidation
Occurs in ER
28
How do CYP450s biosynthesise steroids?
In mitochondria - steroids from cholesterol
e.g. aromatise (oestrogen)
29
What are the biochemical properties of CYP450s?
MW 40,000 - 50,000
Haem as prosthetic group - coordinated to 4Ns of haem tetrapyrrole ring and a cysteine resume at C-terminal, Fe (III)
Haem and oxygen binding regions are conserved but areas for substrate binding vary (conserved within species, but substrate can differ in species)
30
Why is iron specifically in the Fe(III) state in CYP450s?
As Fe(II) is very reactive which forms reactive oxygen species (ROS) causing stress within the cell
31
Why is the cysteine importantly conserved in the sequence?
As it allows binding with the iron species to coordinate it so proper activity takes place
32
What is the general CYP450 reaction with the substrate?
DH + NADPH + O2 -> DOH + NADP + H2O
33
What are the main components required for CYP450s to react?
CYP450 enzyme, NADPH-CYP450 reductive and phosphatidylcholine
34
What are the properties of the NADPH-CYP450 reductase enzyme?
Essential in electron transfer
Flavoprotein containing flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN)
Electrostatic interaction with CYP450 enzyme using carboxyl groups on reductase and amino groups on CYP450s
35
How do mitochondrial P450s conduct electron transfer?
Using adrenotoxin
36
What is the reaction mechanism of CYP450s?
P450+Fe3+
↓Reduced drug added
P450+Fe3++D-H
↓e- added
p450+Fe2++D-H
↓O2 added
P450+Fe2+-O2+D-H
↓e- added
P450+Fe2+-O2-+D-H
↓2H+ added and converted to H2O
P450+Fe3+-O+D-H
↓D-OH produced
P450+Fe3+ reformed
37
What intermediates are used to donate the electron to the Fe3+ in the CYP450 reaction?
FAD and FMN from their semiquinone and reduced forms to semiquinone forms
38
Why are flavins used to donate the electrons to the CYP450?
As the NADPH does not interact directly with the CYP450
39
How are CYP450s induced and what are their properties?
Exposure to certain xenobiotics
Represented by increase in mRNA and protein
Induction of different isoforms
Inducers: Polycyclic aromatic hydrocarbons (PAH) and barbiturates
40
What is the effects of CYP450 mediated reactions?
They inactivate many drugs and increase rate of excretion
They activate carcinogens (tobacco smoke)
Toxic molecules from harmless drugs
Activate prodrugs
41
What is the tissue distribution of CYP450s?
Highest in liver
Lower levels in kidney, lung, intestine, adrenals and brain
Detected mainly in extrahepatic tissue
42
What are the main examples of reactions catalysed by CYP450s?
N-Dealkylation
O-Dealkylation
Aliphatic hydroxylation
Aromatic hydroxylation
43
Where are the CYP450 isoforms encoded?
From different genes
44
What are the properties of the CYP450 isoforms?
Distinct substrate specificity overall of metabolism using several CYP450s
Essential knowing with CYP450 metabolises the drug before therapeutic use
45
What are the main 3 CYP450s?
CYP3A4
CYP2D6
CYP2C9
46
Why are there only 3 main CYP450s?
They are responsible for the majority of metabolism (~90%)
47
How can specific P450 isoforms responsible for specific drug metabolism be identified?
Correlation analysis using liver microsomes bank
Inhibition studies (chemical and antibody inhibition)
Studies using purified or expressed enzymes, wide range available
48
How are P450s identified using correlation analysis using human liver microsomes?
A drug is incubated with human microsomes and level of metabolite is measured
Probes are used to detect the specific P450
Correlation analysis is used (probe vs substrate) an R^2 values s produced describing the correlation, higher R^2 values equate to more probable enzyme for the substrate
49
How are P450s identified using inhibition studies chemically?
Incubation of drug with human liver microsomes and NADPH
A specific CYP450 inhibitor is added
Measurement of metabolite formation is collected
This value is then compared to the control
50
How are P450s identified using antibody inhibition?
Drug is incubated with human microsomes and NADPH
Antibodies are added against the specific P450
Metabolite formation is measured
This value is compared to the control
51
How are P450s identified using purified/expressed enzymes?
They are transfected into a cell line with high expression of a specific P450
The drug is then added and metabolite formation is measured
52
What is the substrate for CYP1A1?
Benzo[a]pyrene (environmental toxins e.g. PAH)
53
What is the substrate for CYP1A2?
Caffeine
Heterocyclicarylamines
54
What is the substrate for CYP1B1?
Benzanthracene (has similar substrates to CYP1A1)
55
What are the substrates for CYP2A6?
Courmarin (organic chemical, anti-inflammatory, antioxidant and anti microbial properties)
56
What are the substrates for CYP2B6?
Cyclophosphamide (procytotoxic drug)
57
What are the substrates for CYP2C8?
Taxol
Retinoic acid (metabolite for vit A)
58
What are the substrates for CYP2C9?
Polymorphic enzyme so not always fully expressed (need titration of medications esp. warfarin)
Warfarin (narrow TW has implications on vit K production)
Ibuprofen
59
What are the substrates for CYP2C19?
Extremely polymorphic, often little activity in individuals
Omeprazole
Clopidogrel
60
What are substrates for CYP2D6?
Also polymorphic but has little effect on metabolism
Codeine
Metoprolol
Tamoxifen - chemotherapy drug used for breast cancer
Etc.
61
What are the substrates for CYP2E1?
Ethanol - has implications when taken in converse with paracetamol
Carbon tetracholride
62
What are substrates for CYP3A4?
Nifedipine
Cyclosporine
Many others
63
What are the properties of CYP2D6?
Substrates have basic nitrogen ionised at physiological pH and hydrophobic region
Oxidation usually 0.5-0.7nm from basic nitrogen
5-10% population lack enzyme due to mutation
Substrates particularly CV agents, antipsychotics or antidepressants
Hydroxylation common
Not inducible
64
What are the properties of CYP2C9?
Substrates have high hydrogen bond forming potential, ion par formation 0.5-1nm from site of metabolism
Substrates wide variety of drug types but especially NSAIDs
Subject to polymorphisms
Inducible by barbiturates and rifampicin
65
What are the properties of CYP3A4?
Highly inducible by glucocorticoids, rifampicin and others
Structural diversity in substrates, binding side accommodates large molecules and binding is mostly hydrophobic
N-dealkylation reactions common but also aromatic hydroxylation
Effects on genetic polymorphism more limited than for other P450s
66
What are the other P450 isoforms important for drug metabolism?
CYP2C19
CYP1A2
CYP2B6
CYP2C8
CYP2A6
67
What is the mechanism of imipramine metabolism?
CYP3A4, CYP2C19 and CYP1A2 demethylate to Desmethylimipramine then hydroxylation to be conjugates and excreted (40%)
CYP2D6 hydroxylates imipramine to be conjugates and excreted (25%)
CYP3A4 Dealkylates and CYP2D6 allows conjugated then excretion (15%)
CYP3A4 can dealkylate imipramine to excretion (13%)
68
Which factors determine metabolism using specific P450s?
Topography of active site of enzyme
Degree steric hinderance of access of Fe-O complex to possible sites of metabolism in substrate
Ease of electron or hydrogen abstraction from various carbons or heteroatoms of the substrate
69
How can tolbutamide metabolism be modified?
Methyl group removed so metabolism using CYP2C9 no longer takes place
70
What are the oxidation non-P450 phase I metabolism reactions?
Flavin-linked monooxygenases
Prostaglandin H-synthase-dependent co-oxidation
Amine oxidases
Oxioreductases
71
What are the hydrolysis non-P450 phase I metabolism reactions?
Esterases
Epoxide hydrolases
72
What are flavin-linked monooxygenases?
Multienzyme family
Smaller but similar to P450s
Require NADPH and O2
Overlap substrate specificity with P450s but distinct metabolites
73
What is the FMO reaction mechanism?
FMO-FAD
↓NADPH + H+
FMO-FADH2 (NADP+)
↓O2
FMO-FADHOOH (NADP+)
↓Monooxygenated product produced
FMO-FADHOH(NADP+)
↓NADP+ and H2O produced
FMO-FAD
74
What are the properties of FMOs?
Oxidise nucleophilic nitrogen, phosphorus or sulphur
Contain FAD as prosthetic group
Located in ER
5 different isoforms (50-55% identity)
FMO5 said to be only inducible one (using rifampicin in hepatocytes)
75
How to FMOs change N-deacetyl ketoconazole?
N-hydroxylation
76
What is tamoxifen metabolised to using FMO1?
Tamoxifen-N-oxide
77
Where is the most FMO1 found?
Kidney
78
Where is FMO2 found?
Liver and kidney
79
Where is FMO3 found?
Liver in abundance but unknown in kidney
80
What is peroxidase-dependent cooxidation?
Xenobiotics undergoing oxidation during physiological peroxidase reactions
81
What are the 3 different enzymes involved in peroxidase activity?
Prostaglandin H-synthase (COX enzyme)
Myeloperoxidase
Lactoperoxidase
82
What is the function of the prostaglandin H-synthase enzyme?
Co-oxidation of arachidonic acid to prostaglandin and xenobiotic to metabolite
COX activity (arachidonic acid to PGG2) (oxidation)
Peroxidase activity (PGG2 to PGH2) (reduction)
Therefore redox reaction
83
How does prostaglandin differ to P450s and FMOs?
It does not require NADPH and O2 as a cofactor to transfer O from xenobiotic
84
What are amine oxidases?
Catalyse oxidation of monoamines, diamines and polyamines
Monoamine oxidase is mitochondrial oxidises using FAD pathway
MAO in metabolism of NTs but also some drugs
85
What are the different monoamine oxidases?
A and B
86
What are the different reaction mechanisms of monoamine oxidase?
RCH2NH2 + FAD → RCH=NH + FADH2
RCH=NH + H2O → RCHO + NH3
FADH2 + O2 → FAD + H2O2
87
What are the oxidoreductases?
NAD(P) dependent enzymes in oxidising direction, NAD(P)H dependent in reducing direction
88
What are examples of oxidoreductases?
Alcohol dehydrogenase
Aldehyde dehydrogenase
Carbonyl reductase
NAD(P)H quinone oxidoreductase
89
What is the cytosol?
Everything but membrane bound organelles
90
How do both alcohol and aldehyde dehydrogenase metabolise drugs?
Alcohol:
RCH2OH + NAD+ → RCHO + NADH + H+
Aldehyde:
RCHO + NAD+ → RCOOH + NADH + H+
91
What is the function of alcohol dehydrogenase?
Used in ethanol detoxification (CYP2E1)
NAD+ required
Alcohols to carbonyls
92
What is a disadvantage of aldehyde dehydrogenase?
It is easily saturated
93
What is carbonyl reductase?
Catalysed variety of carbonyl compounds (e.g. quinones, prostaglandins, menadione, etc.)
94
What reactions do carbonyl reductases catalyse?
R-CHOH-R' + NADP+ ↔ R-CO-R' + NADPH + H+
95
How is doxorubicin metabolised?
Using carbonyl reductase
96
What is quinone oxidoreductase?
2 e- reductase, catalyses reduction of multiple substrates
Quinones to hydroquinones (no semiquinone as intermediate)
Detoxification of quinones, quinone-mines and ado compounds
97
What are the properties of esterase phase I enzymes?
Found in plasma, liver, etc. cytosolic and microsomal forms
Number of different forms
98
What are the different forms of esterase enzymes?
Butyrylcholinesterase - aspirin
Carboxylesterase (CES1)(hCE1) - cocaine and heroin
Carboxylesterase 2 (CES2)(hCE2) - procaine
Paraoxonase (PON1) - arylesters, 3=statin prodrugs
99
What are the differences between carboxylesterases?
Esters, thirsters and amides
CE1 - small OH, large acyl
CE2 - large OH, small acyl
100
What are the mechanisms of both cocaine and heroin hydrolysis?
Use hCE1
Cocaine → benzoylecgonine
Heroin → 6-acetyl morphine → morphine
101
What are the epoxide hydrolases?
Specialised type esterase, in presence of water epoxides are cleaved
2 different forms - microsomal and cytosolic
Benzo[a]pyrene activation and metabolism of drugs
102
Why is epoxide hydrolase important in bento[a]pyrene metabolism?
Forms carcinogenic compound allowing binding to DNA which forms adducts
103
What is paroxonase 1?
Multifunctional enzyme - aryl esterase, lactose and paroxonase activity
Mainly in liver but also in plasma
Degredation od oxidised lipids (+ PON2 and 3)
104
What are the different conjugation reactions?
Glucuronidation
Sulfation
Amino acid conjugation
Methylations
Acetylation
Glutathione
105
What enzyme is used during glucuronidation?
UDP-Glucuronsyltransferase
106
What enzyme is used during sulfation?
Sulfotransferase
107
What enzyme is used during amino acid conjugation?
No enzyme required
108
What enzyme is used during Methylation?
Methyltransferase
109
What enzyme is used during acetylation?
Acetyltransferase
110
What enzyme is used during glutathione conjugation?
Glutathione S-transferase
111
What are the aims of conjugation reactions?
Greater MW and more water soluble
Less able to pass through cell membranes
Easier to excrete
112
What is the usual result of phase II metabolism?
Detoxification/detoxication of drugs
113
What is the cofactor required in glucuronidation?
UDP-glucuronic acid
114
What endogenous compounds are glucuronidated?
Steroids, vitamins, bile acids and bilirubin
115
What are the properties of UDP-GTs?
Only in endoplasmic reticulum
Monomeric - MW 50-60000
No prosthetic group
Highest in liver also in kidney, lung, small intestine, skin and adrenal gland
Large number of different isoforms
116
Why is it beneficial for all main phase I and II enzymes to be in the same place?
Not reactive as they are not wandering around the cell
117
What are the effects of glucuronidation?
Normally promotes excretion and loss in biological activty
Some glucuronides have biological activity
Drug glucuronides are reactive and bind irreversibly to small proteins - triggers immune response (changes protein overall structure)
118
What are the families of UDP-GT genes?
UGT 1 and UGT 2
10 isoforms of UGT 1
119
What enzymes are UGT 2 isoforms similar to?
CYP450 enzymes
120
What are the substrates of the UGT1A1 isoform?
Bilirubin and ethinyl estradiol
121
What are the substrates of UGT1A4?
TCA antidepressants
Imipramine
Amitrypyline
Chlorpromazine
122
What are the substrates of UGT1A6?
Paracetamol
123
What are the substrates of UGT1A9?
Valproate
Naproxen
124
What are the substrates of UGT2B7?
Morphine and ibuprofen (2B also metabolise steroids)
125
How are glucuronide conjugates eliminated if they are more or less than 400kDa?
<400 - urine
>400 - bile
126
What can happen to glucuronide conjugates in the bile?
They undergo enterohepatic recirculation so they can be potentiated and cause more pharmacological activity (beta-glucuronidase)
127
What are the different things UDP-glucuronic acid react with?
Alcohols
Carboxylic acids
Amines
Amides
Thiols
128
How can UDP-GT be induced?
Exposure to xenobiotics - not a notable increase
Family 1 - phenobarbitone and polycyclic aromatise hydrocarbons (not specifically)
Specific response through antioxidant response element
129
What are the properties of sulfotransferases?
Cytosolic enzymes - 2 subunits (MW 34000 each)
Wide tissue distribution
11 isoforms known - endogenous substrates
130
Which drugs does SULT1A1 metabolise?
Phenolic compounds, paracetamol and N-hydroxy PhIP (well-cooked meats)
131
Which drugs does SULT1A3 metabolise?
Phenols and 1-hydroxy methylpyrene
132
What drugs does SULT1C2 and 4 metabolise?
Phenols, N-hydroxy-2-acetylaminofluorene
133
What is the co-substrate required for sulfotransferase reactions?
PAPS (3'-phosphoadenosine 5'phosphosulphate) - source of sulphate
134
What are the properties of sulfation reactions?
Sulfate esters much more soluble - anionic
Pharmacologically inactive
Reduced availability of drugs
135
When is sulfation most likely to take place?
At low substrate concentrations (availability is limited)
136
Why is sulfation least likely to take place?
Because PAPs is an expensive commodity
137
What is a disadvantage of the use of sulfation for metabolism?
Can result in activation of carcinogens
Some sulfoxy derivatives of pro-carcinogens are unstable so spontaneously form reactive nitrenium
138
How are sulfotransferases regulated?
Metabolites of nuclear aryl hydrocarbon receptor (AhR) ligands (dioxins) are substrates for SULT
139
Where does amino acid conjugation take place?
In the mitochondria
140
What are the major amino acids in conjugation?
Glycine, taurine and glutamine
141
How does amino acid conjugation take place?
Activation carboxyl group to acyl-coenzyme A derivative then amide formation with amino group conjugating to amino acid
142
What is the initial amino acid reaction carried out by?
Medium chain fatty acid: CoA ligases (MACs)
143
What happens when MACs form with glycine N-acyltransferases?
Conjugates on both benzoic acid and salicylic acid
144
Why are there so few xenobiotics conjugated using amino acids?
Because the glycine N-acyltransferase is selective
145
Which drugs get metabolised via acetylation?
Compounds containing: amino, hydroxy and sulfhydryl groups
146
Which enzyme carries out acetylation?
Acetyltransferases
147
What donates the acetyl group?
Acetyl coenzyme A
148
Where have the acetyltransferases been detected?
In the cytosol (NAT1 and NAT2)
149
How were acetyltransferases first discovered?
In examples of pharmacogenetics (slow acetylation then links with bladder cancer)
150
What are the properties of acetyltransferases?
N-acetyltransferases but also transfer acetyl group to hydroxy
Main NAT in cytosol
Small proteins - products of adjacent genes
NAT1 in most tissues NAT2 mainly liver and intestine
Distinct but overlapping substrate specificities
NOT INDUCIBLE
151
What are the differences between genetics in the N-acetyltransferase genes?
NAT2 is reffered to as polymorphic - unable to acetylate certain compounds
NAT1 is monomorphic as it is much more active and can still metabolise most compounds
152
What are important examples of drugs metabolised via NATs?
Isoniazid(tuberculosis) - NAT2
Sulfamethoxazole(antibiotic) - NAT1
153
What does it mean if a reaction is exergonic?
There is a release of energy
154
What are the effects of acetylation?
Masking of amine group with acetyl group - therefore decreased solubility
Activate certain pro carcinogens (aromatic arylamines - well-cooked meat)
Deacetylation can take place using esterase enzyme
155
How can bladder cancer occur from acetylation?
As the metabolised compounds pass through the bladder they can become deacetylated with can cause some carcinogens to reactivate
156
How is there little association between NATs and cancer?
As cancers are multigenerational diseases
DNA repair enzymes can fix the adducts in DNA
157
How can cancer be caused by NATs (supposedly)?
Activation using NATs can produce reactive metabolites which initiate carcinogenesis
158
What is glutathione?
Tripeptide importan in maintaining reduced environment within cell
159
What are the different versions of glutathione?
Reduced (GSH) and oxidised (GS-SG)
160
Where are glutathione-S-transferases mainly found?
In the cytosol as soluble enzymes
161
What is the approximate Mr?
25000
162
How are conjugates made using GST?
Conjugate reduced glutathione to electrophilic compounds through nucleophilic cysteine thiol group
163
Where are GSTs found?
In most human tissues
164
What are the different classes of GSTs?
Alpha (GSTA1-A4)
Mu (GSTM1-M5)
Pi (GST-P1)
Theta (GST-T1, GST-T2)
165
What is the theta class most specific for?
Small molecules
166
What can the different subfamilies of GST do?
They can form heterodimers with proteins from the same subfamily only
167
What are the 2 different common genetic polymorphisms in GSTs?
GSTT1 and GSTM1
168
Which glutathione classes are inducible?
Alpha and mu
169
What are the ranges of GST inducers?
Polycyclic aromatic hydrocarbons, barbiturates and antioxidants
170
Which phase II enzyme is thought to be protective against carcinogens when induced?
Glutathione
171
Where does glutathione bond?
Through nucleophilic cysteine thiol group
172
What is GST reactive with?
electrophilic substrates
173
What are the different groups glutathione substitutes?
Acetates, sulfates, nitrates and epoxides
174
What do conjugates of glutathione lose?
Glutamic acid and glycine
175
What derivatives are formed in GST conjugation?
Mercapturic acid
176
What are the different types of drug substrates?
Anticancer (cyclophosphamide)
Vasodilator (nitroglycerine)
Analgesic (paracetamol)
Diuretic (ethacrynic acid)
177
When is GST involved in paracetamol conjugation?
From the conversion of NAPQI using CYP450s then using GST to convert to a non-toxic metabolite
178
Which GST class is used in paracetamol conjugation?
Pi
179
What does NAPQI cause when GST is depleted?
Allows covalent binding to proteins in centrilobular regions
Causing death and necrosis
180
What does methylation and acetylation do to drug metabolism?
Doesn't improve solubility
Reduction of pharmacological activity
181
What co-factors allow methylation and acetylation?
Methylation - S-adenosylmethionine (SAM)
Acetylation - Acetyl CoA
182
Which groups can be acetylated and methylated?
Methylated: amines, thiols, alcohols
Acetylated: amines, amides, hydrazines
183
What are the different types of methyltransferases?
O-methyltransferases (catechol and phenol)
N-methyltransferases (histamine and nicotinamide)
S-methyltransferases (thiopurine and thiol)
184
How do methyltransferases work?
Increase lipophilicity via methlyation
185
How is inorganic arsenic metabolised?
GSH conjugation and oxidative methylation to MMA(III) and DMA(III)
186
How is arsenic toxic?
Can act as phosphate meaning it uncouples hydrolysis and inhibits enzymes
