Lecture 4 - Toxicokinetics Flashcards
Phase I metabolism reactions
Phase I includes 2 possibilities:
1 - oxidation: formation of an epoxide - TOXIFYING (electrophile-will lose e- and look for one)
2 - hydrolysis: addition of an OH group is DETOXIFYING (nucleophilic - they will donate e-)
Phase II metabolism reactions
Phase II rxns are conjugation rxns:
- glutathione conjugation (ELECTROPHILES)
- sulfartion, acetylation, glucoronidation (DONE TO NUCLEOPHILES)
Which phase I reactions introduce a functional group?
-reduction
Oxidation
Hydrolysis
These functional groups are then used in conjugation rxns (to add onto another group)
Hydrolysis (Phase I)
- Epoxide hydrolase will add a water to the epoxide to form a diol (which acts as the detoxification product)
- Therefore hydrolysis rxns can remove epoxides while oxidation rxns make epxoides
Reduction (Phase I)
Reduction of Quinone:
-Quinone —> Hydroquinone via oxireductase
This will increase affinity for hsp90, greater potency as chaperone inhibitor
-Quinone –> Semiquinone via cytochrome p450/b5 reductase will lead to ROS and toxicity –> semiquinones have been linked to leukemais and lymphomas
Oxidation (Phase I)
Ethanol is mainly metabolized via oxidation rxns:
-alcohol dehydrogenase (creates acetyladhalde)
-acetaldehyade dehugroenase (creates ascetic acid)
Forms metabolites that are more readily excreted and less lipid soluble thus do not fit/pass membranes, get excreted
*note: ethanol is not only metabolized by oxidation rxns
What are the 3 ways by which ethanol is metabolized?
- Oxidation (ADH, ALDH)
- By CYP450s (CYP2E1) –> can make actvie metabolites, which is problematic
- Peroxisomes (CATAALSE enzyme)
3 diff pathways, all with diff effects
CYP450s
One of the major enzymes for redox (reduction/oxidation) rxns are CYP450s. They have a heme and protein portion and a reductase, also many factors. They CATALYZE ELECTRON TRANSFER RXNS ! ~AS MANY CYP450s as antibodies.
CYPs are found in lyric cells and in the ER (endoplasmci reticulum). They have a hydrophobic region which allows them to anchor into the membrane.
How do the e- transfer rxns work?:
1. Toxin binds heme
2. Heme portion does fe3+ –> fe2+ (ferric to ferrous) via NADPH
3. O2 comes in (ternary compelx)
4. Electron comes in again to split O2 into water
5. GET formation of hyrolatxed chemical that can be released (hydrolyssi rxn)
Thus involves o2, cofactors, reductase, in order to complete 2 e- transfer rxns
Metabolism of Benzo[a]pyrene
- Benzopru3yene is a PAH (smile, grilled meat, etc)
- not very toxic on its own! Can be metabolically axctviated to become dihydrodiol epoxide, aka ultimate carcinogen. Can also make another epoxide in a diff pathway using a diff CYP450, which does not recycle back to ultimate cacrcinogen.
- if the ultimate carcinogen is made, get:
- a dihydridiol epoxide that is very reactive and electriphilic, causing ROS
- cannot be metabolized by epoxide hydrolase
- covanrlty bind DNA
- mutate oncogenes
- case lng and skin tumours –> one of the factor involved in probability of getting skin and lng cancer is the relative % of each CYP450 that does the bad or less bad rxn
Phase II: which functional groups must be activated in order for a reaction to proceed?
-an amino acid like glycine
-sulfate
-acetyl
-glutathione
-methyl
-gkucoronides
These all required a delivery mechanism for the conjugation to occur (an activation that allows the rxn to proceed)
Glucoronidation
-chloramphenicol can be used as a delivery mechanism to attach glucoronide to a hydroxyl group - the glucoroniec can be placed in diff locations (aka it can act as a transferase)
Acetylation
-Can add an acetyl group to amine groups, isoniazid
-This is done by N-acetyl transferases 1 and 2 (NAT1/2 –> each have diff substrates)
NAT2 SLOW: INCREAES BALDDER CANCER IN HEAY SMOKERS
Glutathione Conjugation
Glutathione=cysteine, glutamate, glycine
The cysteine acts as an ucloephile (donates e-) to inactivate reactive electrophiles using glutathione transferase (transferase)
Afterwards, can add an acetyl to increase excretion
THUS, ELECTROPHILIC COMPOUNDS ARE CONJIGATED VIA GLUTHATHIONE
Metabolic Activation of Acetaminophen
Acetaminophen can be metabolized by CYP450s and then adding a sulfate or glucoronide
The cyp450S produce metabolic reactive oxygen species
If the sulfation and glucoronidation capacities are used, gluthathione kicks in (b/c gluthathione is what metabolizes electrophilic ROS)
If gluthathione capacity is saturated –> free radicals damage liver, kidney, and other organs
AVD for warfarin, ethanol, chloroquine
- warfarin: low (bound to plasma so stays in serum)
- ethanol: high (~ body water)
- chloroquine: very high
