Metabolism Flashcards
What is biotransformation
The enzyme-catalyzed conversion of one xenobiotic into another
What is detoxification
biotransformation results in a LESS toxic metabolite
What is bioactivation
biotransformation results in a MORE toxic metabolite
Why is it essential for biotransformation (or metabolism) of xenobiotic to more water-soluable metabolites
To terminate their biological activity and eliminate them from the body
What do biotranformation enzymes convert lipophilic xenobiotic into
highly water-soluble metabolites that are easily excreted from the body (mainly in urine and to a lesser extent bile/feces)
Biotransformation reactions phase 1
biotransformation enzymes modify the xenobiotic molecule mainly by oxidation (eg. addition of an -OH [hydroxyl] group to the xenobiotic)
[Oxidation, Reduction, Hydrolysis, Hydration, Dehalogenation]
Phase 2
synthetic reactions that conjugate the xenobiotic with highly polar endogenous compound in the cell (eg. a carbohydrate, sulphate, or acetate)
[Sulphation, Glucuronidation (add glucose), Glutathione conjugation, acetylation, amino acid conjugation, methylation]
Which organ is the most important site of xenobiotic biotransformation?
The liver is the most important, Tissue localization of xenobiotic-metabolizing enzymes
Relative amount - Tissue
high - Liver
medium - Lung, kidney, intestine
Low - Skin, testes, placenta, adrenals
Very low - Nervous system
Cytochrome P450-dependent monooxygenases
CYPs; aka mixed-function oxidases - are the major Phase 1 oxidative enzymes
Reactions in phase 1 usually involve adding what
adding or exposing a polar functional group (eg. -OH, -COOH, -NH2) to lipophilic xenobiotic molecule
CYP enzymes that biotransform xenobiotics are located on what
smooth endoplasmic reticulum
what does monooxygenase indicate
that they catalyze the insertion of an oxygen atom into the xenobiotic molecule, ie the simple hydroxylation reaction:
xenobiotic (R-H) + O2 + NADPH -> Metabolite (R-OH) + H2O + NADP+
CYP’s are also involved in which two reactions
a wide variety of catabolic (degradation) and anabolic (synthesis) reactions - involving endogenous compounds ie steroid hormone
> 100 different CYP enzymes have been identified (designated in families such as…)
CYP1, CYP2, CYP3, subfamilies CYP1A, CYP2E, CYP3A and specific enzymes CYP1A2, CYP1E2, CYP3A4 based on the DNA sequence similarity of genes coding for enzymes
CYPs are very versatile and unique enzymes due to…
broad and overlapping substrate specificities:
Broad:
Overlapping:
Broad: one enzyme can biotransform many xenobiotic
Overlapping: One xenobiotic can be biotransformed by several enzymes
CYPs usually ______ (_____) a xenobiotic but in certain cases can ________ a xenobiotic to a more pharmacologically or toxicologically active metabolite
CYPs usually inactivate (detoxify) a xenobiotic but in certain cases can a xenobiotic to a more pharmacologically or toxicologically active metabolite
CYP Basic Hydroxylation Reaction
R-H + O2 + NADPH -> R- OH + NADPH + H2O
R-OH = Hydroxylated metabolite
H2O = Byproduct
Gets oxidized by single hydroxyl group
Add a hydroxyl group to:
RCH2CH3
OH
|
RCHCH3
Many enzymes involved in steroidogenesis are CYPS
-Produces steroids/hormones
- regulate physiological things
-regulates water balance
- all have these genes, transcription, & expressions of these genes determine these physiological functions
What is the hepatic portal venous system
delivers all substances absorbed from the GI tract to the liver before they reach the systemic circulation and be delivered to the rest of the body (to exert effects)
What is the first pass effect
vertebrate animals evolved to detoxify to survive against ingesting lethal substances through plants
Can result in nearly complete inactivation (>90%) of certain drugs (and xenobiotics) after oral ingestion
First pass effect; liver cycle
Drugs that are absorbed from the gut may be biotransformed by enzymes in the gut wall and liver before reaching the systemic circulation. This process lowers their degree of bioavailability, cycle of reabsorption
Oral bioavailability
The fraction of an orally administered xenobiotic that reaches the systemic circulation in an unchanged form
bioavailability = AUC oral/ AUC IV
AUC = area under the curve
The major increase in water solubility (and thus excretability) of xenobiotic occurs after which phase
after phase 2 reactions, which add (conjugate) a large water-soluble group to an existing polar functional group on the molecule
Glucuronidation, what enzyme does it require
where is it located
The major phase 2 biotransformation pathway in mammals, requires enzyme UDP-glucuronosyl transferase (UGT) and co-factor UDP-glucuronic acid
located on smooth ER membrane
Where is majority of phase 2 enzymes located
located in the cytoplasm, the glucuronidation is located in smooth ER membrane
Sulfation
Enzyme sulfotransferase (ST) and cofactor PAPS
Acetylation
Enzyme N-acetyltransferase (NAT) and cofactor actyl coenzyme A
Phase 1 > ______>Phase 2
phase 1> hydroxylated group > phase 2
GST
Glurathione S-Transferase = enzyme
abundant enzyme - approx 5% of cytosolic protein in liver cells
GSH
Glutathione - cofactor
- detoxifies
- most potant antidonte
review
Phase 1 - bioactivating or detoxification
oxidation, reduction, hydrolysis, hydroxylation
Review phase 2 - bioactivation or detoxification
conjuation
Biotransformation: which of the following 6 pathways are good or bad
Foreign compound - Stable metabolite - Excretion
constant GOOD effects
Foreign compound - Toxic/reactive metabolite - metabolism/reaction with protective agents - stable metabolite - excretion
bad to toxic reaction
good to stable metabolite
Foreign compound - Toxic/reactive metabolite - Reaction with critical targets - Toxic effect (A)
constant bad effects
Foreign compound - stable metabolite - toxic effect (A)
Good effect to stable metabolite
Bad effect to toxic effect
Foreign compound - stable metabolite - Excretion
Constant good
Foreign compound - toxic effect
constant bad
Genetic and environmental factors influencing biotranformation
enzyme induction and inhibition
- CYP enzymes and phase 2 enzymes can all be induced and inhibited
What is induced
what in inhibited
induced - increased activity
inhibition - decreased activity
Infraspecific differences
genetic differences - different forms in the expression of enzymes
can result in a subset of the population being “poor metabolizers” or “rapid metabolizers”
ADH in humans
Alcohol dehydrogenase (ADH)
influences of biotransformation
- enzyme induction and inhibition
- intraspecific differences
- interspecific differences
- sex and age
- Diet (nutritional factors)
- Disease (underlying pathology)
