Drug metabolism Flashcards
Outline the requirements for CYP mediated oxidation reaction (phase I)
Drug
Cofactor for reduction (NADPH/NADH)
Molecular oxygen
Protons
What are the products of a CYP ocidation reaction
Oxifised drug
NADP+
H2O
What is the common structure of cytochrome enzymes
Porphyrin ring with iron in the middle CATALYTIC
Explain how the cyt p450 causes oxidation of a drug
Drug binds at the site of the porphyrin ring containing catalytic iron. Interacts with Fe3+
NADPH or NADH provides an electron. That electron is picked up by iron at the active site. Fe3+–> Fe2+
Oxygen added, which binds at the active site (near the iron and the drug). The electron that the iron gained from the NADPH is now transferred to the oxygen. Iron converts back Fe2+–>Fe3+
Electron makes oxygen aggitated
Another electron is then picked by the iron. Fe3+–>Fe2+
Another electronic rearrangement. Fe3+–>F2+ again. Second electron picked up by the molecular oxygen which is now very unstable
One of the O molecules from molecular oxygen reacts with 2 protons to become water. The other O molecule reacts with the drug to form the hydroxyl group
Substrate released
T/F for the oxidation of the drug with Cyt p450, only 1 NADPH was needed
F…. 2 were needed. 1 molecular oxygen needed
Which molcules can be used as reducing agents in cytp450 reactions
NADPH or NADH (sometimes)
T/F cytp450 involves hydroxylation
T! It is an oxidation reaction BUT
Oxidation reactions generally start with a hydroxylation step
catalysed by the P450 system.
Which step in CYTP450 oxidation is rate limiting
The second reduction
T/f all compounds are oxidised in the same position with CYTP450
f…..
in aliphatic compounds e.g. pentobarbitone (a barbiturate), OH group added in any of the carbons
there is a main metabolite (pictured)
-OH added.
What is acetanilide
It is an aromatic structure
Oxidised by CYTP40
When oxidised forms paracetemol
(-OH added)
but this is not sold as a pro-drug because it is toxic for blood cells
What is N-demethylation
Oxidation of carbon ATTACHED to nitrogen (this tertiary nitrogen acts as substrate for the cytp450)
Oxidised N-methyl group.
Formaldehyde drops off (HCHO), and a proton gained.
e.g. Imipramine (tertiary nitrogen) –>desimipramine
What is O-demethylation
Oxidise carbons attached to oxygen.
Formaldehyde again drops off (HCHO)
e.g. CODEINE TO MORPHINE (in this reaction, happens very well insome people and no tin others due to differing CYTP450)
What is N-oxidation
This is oxidation of the nitrogen group on a teritary nitrogen with lone pair of electrons (trimethylamine)
Lone pair of electron on the nitrogen- it donates it to an oxygen
Makes amine oxide (oxide of tertiary amines)
NOT BY CYTP450 by flavine containing monooxygenase
What occurs in people with low flavine contianing monooxygenase
This is a polymoprhic enzymes so differing amounts
In N-oxidation, the substrate is a tertiary nitrogen with lone pair of electrons.
Smells like fish
Flavine containing mono-oxygenase allows donation of the lone pair of electrons to an oxygen
In people with low levels, the teritiary nitrogen with lone pair accumulates (the trimethylamine), and they smell bad.
FISH ODOUR SYNDROME
Is there a cure for fish odour syndrome
No
Have to avoid trimethylamine
But it’s everywhere including when eating meat in choline which is converted to trimethylamine
High rates of suicide etc.
Outline oxidation of ethanol
Ethanol –> acetaldehyde by alcohol dehydrogenase
Acetaldehyde –> acetic acid by aldehyde dehydrogenase
NOT CYTP450
REVERSIBLE
Differentiate ethanol metabolism to most other metabolism
Alcohol dehydrogenase is zero order kinetics.,
Most other enzymes (CYTP450, flavine containing monopoxygenase) are 1st order
T/F CYTP450 is not a reductase
Mostly true, but can act as a reductase very rarely in some circumstances
Where do reductase reactions occur (phase 1)
GI tract by bacterial reductases
e.g. prontosil –> sulphanilamide
prontosil is an antibacteria drug, but of a non antibiotic type. Can’t be used against gut bacteria
T/F oxidation and reduction happen in equal meausre
F… reduction far less common than oxdation
When does hydrolysis of a drug occur
What enzymes
What are the products
On esters or amides
e.g. procainamide
ESTERASES and AMIDASES
Products include COOH and amino groups
T/F xenobiotic metabolism only happens in the liver
F
Phase 1 metabolism can introduce functional groups such as what
–OH, -NH2, -SH or –COOH.
act as handles for phase II metabolism
T/F most of the products of phase I are biologically inactive
T
What is the effect of phase I on drug polarity
Have little effect on drug polarity.
Phase II metabolism involves which reactons and which enzyme
Glucuronidation
(Glucuronyl transferase)
Methylation
(methyl transferase)
Sulphation
(Sulphotransferase)
Glutathione
(Glutathione-S-transferase)
Aminoacid conjugation
(Acyl transferase)
Acetylation
(Acetyl transferase)
What is the aim of phase II metabolis
To put big large endogenous polar molecules onto the drug
Differentiate acetyl transferase and acy transferase
Acetyl transferase involved in acetylation (phase II metabolism)
Acyl transferase involved in amino acid conjugation in phase II metabolism
What is the product of phase II reactions like
Conjugate is almost always pharmacologically inactive
Less lipid soluble
Easier to excrete
What is the most common phase II reaction
Glucoronidation
What is required for phase II metabolism
Enzyme (i.e. the transferase)
+
Conjugating agent
Why does phase II require energy
To make the conjugating agents
List the conjugating agent for each of the phase II reactions
Glucuronidation- UDP-glucuronic acid (UDPGA)
Acetylation- acetyl CoA
AA conjugation- Glycine, glutamine, taurine
Methylation- S-adenosyl-methionine
Sulfation- 3’-phosphoadenosine-5’-phosphosulphate
Glutathione conjugation- glutathione
Which AA are most likely to be used in phase II metabolism
Glycine, glutamine, taurine
What are the target functional groups for each of the phase II
Glutathione: ELECTROPHILES
Acetylation: -OH, -NH2
AA conjugation: -COOH
Methylation: -OH, -NH2
Sulfation: -OH, -NH2
Glucoronidation: -OH, -COOH, -NH2, -SH
T/f sulfation conjugates -SH groups
F…. the target group of sulphation is -NH2 and -OH
-SH groups are targeted by glucoronidation
Why is glutathione important
Because electrophiles, which they target, are going to destroy DNA as this is the most electrophilic biomaterial
Explain the metabolism of ibuprofen
Phase I- OH group added by CYTP450
Phase II- GLUCURONIDATION
UDGPA added with glucucornyl transferase)
Where is the glucuronide group added in Glucuronidation
Glucuronic acid part transferred to
an electron rich atom (N, O or S)
Where are glucuroindes excreted
Often in the bile
Outline acetylation reactions
Amine group, oxygen or sulphur group which was exposed in phase I
Acetyl CoA added
Producted is an acetylated amine
CoA generated
Give an example of acetylation reaction
Sulphanamide
acetyl CoA acts as a donor molecule. Donates acetyl group. Forms acetylated amide
Outline methylation
Target is N, O or S
S-adenosyl methionine acts as donor compound
donates methyl group
Why does methylation occur, even though phase II is meant ot make compounds more waer csoluble and easier to excrete.
Methylation clearly makes it harder to excrete as more lipophiic
Give example
It is a reactionthat occurs endogenously
E.g. NA–> A
The body mistakenly converts drugs with a similar strucutte to adrenalines (e.g. AMPHETAMINE)
Why is sulfation really good
wHAT’S THE PROBLEM
It works opposite to methylation and makes a really polar product
PAPS takes a lot of energy to make (this is the conjugating agent)
But it can make really lipophilic compounds really water soluble
Outline the sulfation
Sulfotransferases catalyse transfer of sulphate (conjugating agent is 3’-phosphoadenosine-5’-phosphosulphate
) to substrates
e.g. -OH, -NH2 are the targets
Give an example of sulfation
Paracetemol. Hydroxyl group on the paracetmol has sulfate group donated by PAPS.
PAP is produced as well as the sulfated derivative
T/F when grinding paracetmol, it would usually dissolve in water
F. Normally very lipopholic
When you give PAPS with it, it will have sulfate donated and become really water soluble and dissolve
Outline conjugaton with glutathione
E.g. halogen or electrophile is the targeted
e.g. benzyl chloride (not drug). glutathione. Thiol groups attacks molecule to form the thiol derivative WITH HCL
What type of molecule is glutathione
Tripeptide
glycine, glutamine and cysteine
What is glutathione used for
Protective factor for removal of toxic compounds for DNA
Which organs have high levels of glutathione
When are levels not high enough
Kidneys and liver
E.g in paracetmol overdose, this system is overwhelmed (NAPQI, an electrophile and derivative of phase I metabolim, needs to be glucoronidated)
