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MBC - Lecture 56

Xenobiotic Metabolism (Miller)

Lecture 56 Xenobiotic Metabolism
Strategy to excrete xenobiotics is to increase ___. solublity
Solubility can be increased by: functionalization, conjugation, and transport
Functionalization Introduce or expose polar group, create a nucleophile, and oxidation and bond cleavage
Conjugation Couple polar group, nucleophile-electrophile reaction
Transport Alter the localization
Cytochrome P450 Membrane-bound heme (Fe) protein complex
Lipophilic substrates of P450 include: endobiotic and xenobiotic
Endobiotic examples: steroids and arachidonic acid
Xenobitiotic examples: food additives, environmental pollutants, and drugs
P450 metabolizes >___% of drugs. >50%
P450 main site of action is in the ___. liver
P450 has ___ substrate specificity. overlapping (i.e. low specificity)
Diazepam has ___ metabolic pathways leading to common end product. multiple
CYP2C19 N-demethylation
CYP3A34 Ring hydroxylation
Xenobiotic metabolism is not an ___ process. efficient
Seldane/Terfenadine Antihistamine, one toxic product & one metabolically active product, caused cardiac arrhythmias/blackouts/deaths, Allegra is improved design
Benzo[a]pyrene has a toxic ___ form. epoxide (electrophile)
The epoxide form ___ DNA. modifies (at the nitrogen bases)
Flavin Monooxygenase (FMO) is ___ bound. membrane bound (microsomal)
FMO contains ___, not heme. flavin
FMO catalytic reactions: oxidation of nitrogen, phosphorous, and sulfur (similar to P450)
N-oxidation of nicotine produces ___. nicotine N-oxide (inactive)
S-oxidation of cimetidine/Tagamet produces ___. cimetidine S-oxide (inactive)
Tagamet is used to treat: (blocks acid secretion from the stomach) ulcers, gastro-esophageal disorder (GERD), and excessive acid secretion
Alcohol Dehydrogenase (ADH) Cytosolic, parenchymal liver cells, converts alcohols to aldehydes
Pyrazole Inhibits ADH
Aldehyde Dehydrogenase (ALDH) Mitochondrial, cytosolic, and microsomal forms, primarily in the liver, oxidizes aldehydes to acids
Disulfiram Inhibits ALDH
ALDH has ___ substrate specificity, broad
UDP-glucuronosyl transferase (UGT) coupling group glucuronic acid
Sulfotransferase (SULT) coupling group: sulfate
Glutathione-S-transferase (GST) coupling group: glutathione
N-acetyltransferase (NAT1/NAT2) coupling group: acetic acid
___ is the major route of conjugative metabolism in most mammals. Glucuronidation
Glucuronidation is catalyzed by ___. UGT
UGT is ___ bound, localized in the ___. membrane bound, ER
Glucuronidation products: O-glucuronides -COOH, N-glucuronides –NH2, S-glucuronides –SH
Conjugation requires ___ group. nucleophilic
SULT Second most important conjugative enzyme, soluble, non-inducible
SULT substrates: steroids, hormones, xenobiotics
Cofactor for SULT: PAPS (activated electrophile), rate limiting
GST catalyzes: glutathione conjugation
GST substrates: steroids, hormone, and few xenobiotics (anticancer)
GST requires ___ cofactor. glutathione
Glutathione in GST serves as the: intracellular reducing agent, non-catalyzed trap for electrophiles, and amino acid tansporter
GST forms highly water-soluble ___ acids. mercaptouric
N-acetyl transferase catalyzes: acetylation
NAT’s polymorphic isoforms are: NAT1 and NAT2
NAT1 is in the ___, and NAT2 ___. liver, colon/other tissues
NAT isoforms catalyze the same reaction but have different: substrate specificity
NAT causes the transfer of an acetyl group, which causes the molecule to: become more polar, but not have a charge
ATP Binding Cassette (ABC) transporter, utilizes ___ to transport. ATP
95% of acetaminophen is broken down to nontoxic metabolites ___ and ___. SULT (65%) and UGT (35%)
<5% of acetaminophen metabolism produces toxic ___, catalyzed by ___. NAPQI, CYP2E1
NAPQI causes: cell death and consumption of GSH (regeneration)
Alcohol consumption induces ___, increasing synthesis of NAPQI. CYP2E1
Created by: emyang



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