Lab 5 Flashcards
What is the purpose of the phase 1 reactions of metabolism?
to introduce or unmask functional groups on a molecule.
the purpose is to introduce polarity for excretion (via the kidneys) AND/OR to form conjugates via phase II reactions
what are the type(s) of enzymes in phase 1 metabolism reactions?
oxidoreductases and hydrolases
major oxidoreductase enzyme system = p450 monooxygenases (CYP enzymes)
the major _______ system is the CYP enzymes (_____)
the major OXIDOREDUCTASE system is the CYP enzymes (P450 MONOOXYGENASES)
what do hydrolases do?
they hydrolyze esters, amides, epoxides, and glucuronides
phase 1 reactions convert __ into ____
lipophilic molecules into more hydrophilic molecules
as mentioned, the drug-metabolism phase 1 reactions introduce or unmask a polar functional group.
give 4 examples
-OH, -NH2, -COOH, -SH
how do the phase 1 metabolism reactions affect pharmacological activity?
can increase, decrease, or leave the pharmacological activity unaltered
where are the cytochrome P-450 enzymes located?
in the endoplasmic reticulum of hepatic (liver) cells
what is the most frequently used enzyme?
cytochromeP450
when are phase 1 metabolism reactions not necessary?
if the drug molecule already contains (polar) functional groups that are suitable for conjugation
as mentioned, phase 1 metabolism reactions are not always necessary.
give a specific example of this
morphine has 2 hydroxyl groups.
Glucuronic acid oxidation of EITHER (or both) is sufficient for excretion.
phase 1 is not needed
give the chemical formula for phase 1 of drug metabolism reactions
Drug + O2 + 2NADPH + H+ —–>(P-450) Drug-OH + H2O + 2NADP+
Phase 2 metabolism reactions consist of what kind of reactions?
conjugation reactions. Primarily detoxification reactions
Phase 2 reactions generally do what?
convert drugs and their metabolites to more polar PRODUCTS for kidney excretion
Glucuronic acid and glutathione form POLAR PRODUCTS
Methylation terminates pharmacological activity
true or false
phase 1 reactions always proceed phase 2 reactions
FALSE
phase 1 is not necessary if a drug molecule already contains functional groups suitable for conjugation (in the case of morphine – glucuronic acid conjugation of either of its 2 hydroxyl groups is suitable for excretion)
_______metabolism primarily undergoes acetylation
isoniazid
is acetylation a phase 1 or phase 2 reaction?
phase 2
what affects the rate of acetylation?
genetic polymorphism
genetic polymorphism affects the rate of acetylation.
slow acetylators are prone to what?
drug toxicity
explain how administration of isoniazid can lead to drug toxicity
phase 1–
when isoniazid undergoes p450 oxidation, it results in a toxic metabolite that is DETOXIFIED by glutathione (GSH) conjugation (phase 2)
Glutathione depletion and drug toxicity can occur when isoniazid is administered at the same time as other drugs that follow the same metabolic pathway and consume glutathione (acetaminophen, sulfamethoxazole, procainamide)
isoniazid cannot be administered with which drugs? why?
acetaminophen, sulfamethoxazole, procainamide
the phase 1 metabolite of isoniazid is TOXIC and is detoxified by glutathione conjugation.
these 3 drugs follow the same metabolic pathway and consume glutathione, which can lead to glutathione depletion and drug toxicity
conjugation reactions are primarily ____ reactions
phase 2 reactions are conjugation reactions - primarily detoxification reactions
what are the targets of phase 2 metabolism?
-drugs with (polar)functional groups that can participate in conjugation
-phase 1 metabolites that are still lipophilic
what are the enzymes involved in PHASE 2 metabolism?
TRANSFERASES (involve the transfer of a suitable moeity)
-N-acetyltransferase (for acetylation)
-glutathione transferase (for glutathione conjugation)
(glucuronic acid enzyme?0
explain what the cytochrome p450 enzymes are
a SUPERFAMILY of mono-oxygenases that catalyze the formation of newly hydrophilic groups such as hydroxyl, amine, or sulfhydryl group on the substrate (drug)
name 2 places where cytochrome p450 enzymes are found
PRIMARILY in the membranes of the endoplasmic reticulum (microsomes) within liver cells
to a lesser extent - in other tissues such as the intestines, kidneys, and lung cells
summarize the reaction that occurs in the cytochrome p450 system
Drug-H +O2 +2e- —> Drug-OH + H2O
Drug-OH = oxidized product
Drug-H = substrate
Explain the properties of cytochrome isoforms
they are expressed by DIFFERENT GENES but have a spectrophotometric absorption peak at or near 450nm when bound and reduced by carbon monoxide.
they have gene similarity or homology (similar protein or nucleic acid sequence)
explain how each cytochrome isoform is named
each uses CYP to represent the cytochrome p450 SUPERFAMILY
followed by a number, a letter, and another number(italics)
each indicates the degree of homology (common amino acid sequence)
true or false
each cytochrome isoform metabolizes the same substrates
FALSE – each metabolizes specific substrates
Why are mammalian CYP enzymes important?
for endogenous metabolism and exogenous metabolism
endogenous - steroid hormones, vitamin D,
exogenous - drugs and xenobiotics
what is a key predictor of drug responsiveness and toxicity?
either expression of the enzyme or alteration in its activity
how can p450 enzymes be induced or inhibited?
through drugs and some food
as mentioned, drugs and some food can induce/inhibit p450 enzymes.
what can these drug interactions do?
they can enhance the toxicity of the drug OR reduce the therapeutic effect of another drug
give an example of a drug that INHIBITS cyp enzymes and a drug that INDUCES cyp enzymes
INDUCER = barbituates. they induce CYP2B which increases the metabolism of drugs, resulting in decreased cost concentration
INHIBITOR = ketoconazole. CYP3A4 inhibitor. REDUCES the metabolism of drugs, resulting in increased drug concentration
give an example of a food that affects CYP enzymes
grapefruit juice contains a CYP3A4 inhibitor, which reduces the metabolism of drugs (like ketoconazole) which significantly increases the drug concentration of CYP3A4 substrates
give 8 examples of drugs that are substrates for CYP3A4.
Thus, their concentration is greatly increased by ketoconazole and grapefruit juice (which contain CYP3A4 inhibitor)
tylenol
cyclosporin A
diazepam
erythromycin
lidocaine
lovastatin
taxol
warfarin
Genetic variants of cytochrome isoforms are due to what?
SNP – a single nucleotide polymorphism
-insertions or deletions of DNA bases
AND
CNVs - copy number variations
what largely attributes to the presence of SNPs? (single nucleotide polymorphism)
individual and population differences in drug metabolism
how is enzyme activity determined?
by specific combinations of genetic variants (ALLELES or HAPLOTYPES)
knowledge of the haplotype combination is used to classify what?
4 different drug metabolizer status types:
- Poor metabolizers – little to no enzyme activity
- Intermediate metabolizers - decreased enzyme activity
- Extensive metabolizers - normal enzyme activity
- Ultra rapid metabolizers - increased enzyme activity compared to extensive metabolizers
true or false
drugs are metabolized differently by different populations
TRUE
in caucasions, the polymorphism for the POOR METABOLIZER phenotype is only seen in 3% of the population as its seen in 20% of the asian population
do drug-drug interactions affect drug metabolism in our bodies?
yes
when drugs share common pathways, enzymes or cofactors will be depleted which will increase drug levels.
the use of alternate metabolic pathways may then result in the production of TOXIC DRUG METABOLITES
When drugs induce enzymes, what happens?
increases enzyme isoform synthesis – affecting more than one P450 isoform and INCREASING THE METABOLISM OF OTHER DRUGS TOO
give 2 reasons why enzyme inhibition would take place.
what may it cause?
enzyme inhibition could be done to increase drug levels or modify/inactivate an enzyme
can cause severe toxicity OR may even be beneficial to reduce the dose and enhance the therapeutic response
what transfers electrons from NADPH to the CYP450-substrate(drug) complex?
Microsomal NADPH-cytochrome p450 reductase (a flavoprotein)
after an electron from NADPH is added (via cytochrome p450-reductase), what happens?
protons (delivered from the cofactor NADH or NADPH) are essential to split molecular oxygen and add a single oxygen atom to the substrate
delivered from NADH or NADPH to the active site in the p450 enzyme
at the end of the schematic for P450 reactions, the substrate (drug) is _______
OXIDIZED
Fe3+ to Fe2+
is this reduction or oxidation?
reduction - gain of electrons
