Flashcards in Lecture 5: Phase II Metabolism Deck (21):
cofactor: urindine-5'-diphospho-D-glucuronic acid (UDP-GA)
Enzyme: Sulfotransferases (ST)
cofactor: 3'-phophoadenosine-5'-phosphosulfate (PAPS)
Enzyme: Glutathione S-transferase
Cofactor: acetyl coenzyme A
Enzyme: methyl transferases
Cofactor: S-adenosylmethionine (SAM)
Amino Acid Conjugation*
Cofactors: taurine, glycine and glutamine
-Glucuronidation is a major pathway of xenobiotic biotransformation in mammalian species, except for the cat family.
-Glucuronidation requires UDP-GA and UGTs, located in the ER of liver, intestine, skin, brain, spleen and nasal mucosa.
-The site of glucuronidation is generally an electron-rich nucleophilic heteroatom (O, N, S).
UGT is a Low Specificity, High Capacity Enzyme
-At low doses of xenobiotic, sulfate conjugates are predominant products.
-At high doses of xenobiotic, glucuronide conjugates predominate
Synthesis of UDP-Glucuronic Acid
The cofactor UDP-GA is synthesized from glucose-1-phosphate and the linkage between GA and UDP has an α-configuration, which protects it from hydrolysis by β-glucuronidase.
Enterohepatic Circulation of Glucuronides
--Xenobiotics conjugated by glucuronides have a β-configuration because of the nucleophilic attack by an electron rich atom on UDP-glucuronic acid, opposite to the linkage between glucuronic acid and UDP. -Enterohepatic circulation delays the elimination of xenobiotics and can increase toxicity.
-Many xenobiotics and endogenous substrates that undergo O-glucuronidation also undergo sulfate conjugation.
-Sulfation occurs through sulfotransferases (SULT)—there are many isoforms of SULTs
-SULTS use phosphoadenosinephosphosulfate (PAPS) as a sulfate donor
Sulfate conjugate excretion
-Most sulfate conjugates are excreted in the urine (actively excreted by organic anion transporters.(OAT)
-Some excreted in the bile may be hydrolyzed by arylsulfatases in gut microflora, which can contribute to enterohepatic circulation of certain xenobiotics. (occurs much less than gluconacidation)
-There are nine genes encoding cytosolic sulfotransferases in humans, and they belong to the SULT1 or SULT2 gene families.
SULT1A1, loss of function is associated with a 3.5-fold increase in esophageal cancer in high-risk males (alcohol, smoking).
-Substrates for glutathione conjugation include an enormous array of electrophilic xenobiotics, or xenobiotics biotransformed to electrophiles.
-Substrates for glutathione S-transferase (GST) share 3 common features: 1) hydrophobic; 2) electrophilic; 3) react nonenzymatically with glutathione (GSH) at a measurable rate.
-The concentration of GSH is very high in liver (10 mM) and GST makes up 10 % of total cellular protein.
-GSH is the co-factor for GST
-Aflatoxins are naturally occurring mycotoxins that are produced by many species of Aspergillus, a fungus.
-They can be found on moldy peanuts, corn and other crops.
-Aflatoxin B1 is the most potent liver carcinogen.
GSTs are dimers composed of identical subunits of 23-29 kDa, although some form heterodimers. 95 % are soluble and 5 % are microsomal.
2.Soluble (4 Classes)
3.A: GSTA1 formerly
called ligandin; (basic pI)
-M: neutral pI
-P: acidic pI
-T: one enzyme
Excretion of Glutathione Conjugates
Glutathione conjugates can be formed in the liver and can be excreted intact in bile or can be converted to mercapturic acids in the kidney and excreted in the urine.
-N-acetylation of xenobiotics is performed by N-acetyltransferases (NAT)
-N-acetylation is a major route of biotransformation for xenobiotics containing an aromatic amine (R-NH2).
-Unlike other Phase II reactions, acetylation masks an amine with a nonionizable group and are less water soluble than the parent compound.
-NAT uses the co-factor acetyl-Coenzyme A (acetyl CoA)
-There are two N-acetyltransferases NAT1 and NAT2
Polycyclic aromatic amines:
-2-Naphthylamine (BNA) is an aromatic amine used to make azo dyes. It is a known human bladder carcinogen and has largely been replaced by less toxic compounds.
-BNA also is present in cigarette smoke