EXAM 5 - Cofactors for Phase 2 Metabolism 1 Flashcards
All Phase 2 enzymes require cofactors - this deck goes over the reactions that occur between the enzymes and their cofactors (SPECIFICALLY GLUCURONIDATION)
During glucuronidation, where is the electrophilic site? What occurs there?
Electrophilic site on glucuronic acid
* The substrate (drug) binds to the electrophilic site
During sulfation/sulfonation, where is the electrophilic site?
Electrophilic site on sulfate group
During acetylation, where is the electrophilic site?
Electropilic site on acetate group
During methylation, where is the electrophilic site?
Electrophilic site on methyl group of sulfonium ion.
During amino acid conjugation, where are the nucleophilic sites?
Nucleophilic site on NH2 of each amino acid.
During glutathione conjugation, where is the nucleophilic site located?
Nucleophilic site on SH of cysteine
* entire molecule of glutathione gets added to the substrate
How many UGT gene families are there?
2 gene families: UGT1 and UGT2
* each with many UGTs
* all microsomal (embedded in the ER membrane)
Describe the process of UDP-GA synthesis (UGT cofactor).
- starts as glucose
- enzyme uses UTP to take off 2 phosphate groups
- oxidized to glucuronic acid –> UDP-GA
- transfered across the ER to the lumen of the ER
Explain the process of glucuronidation of a nucleophilic drug (-OR) by UGT.
- the nucleophilic drug has a slightly negative charge on the oxygen, which attacks the carbon (slightly pos. charge) on the glucuronic acid portion of the cofactor UDP-GA
- the cofactor gets transferred onto the -OR of the drug (excludes UDP portion –> only glucuronic acid)
- -OR replaces UDP –> UDP leaves and reenters the cytoplasm to be recycled as UTP
- inversion of configuration from downwards to upwards
- product: b-glucuronide (glucuronic acid + substrate)
Describe the product of OH conjugation by glucuronic acid.
Glucuronidation of alcohol forms ether
* ETHER: R-O-R’
* oxygen on the alcohol groups attacks the electrophilic site of glucuronic acid
Describe the product of acid conjugation by glucuronic acid.
Product: esters
* nucleophilic oxygen on the acid is going to attack the UDP-GA
* ESTER =
Explain what groups substrates must have in order to get glucuronidated.
- OH
- COOH
- NH2
- SH
- C–C
Can glucuronidation form toxic products?
Yes.
Describe what a glucuronide is.
- The product of glucuronidation
- a substrate that is linked to glucuronic acid
Describe what a glucuronide is.
- The product of glucuronidation
- a substrate that is linked to glucuronic acid
Describe when glucuronides end up in the urine versus the bile.
Glucuronide MW..
MW > ~400 –> bile
MW < ~300 –> urine
Describe the UGT1 family gene structure.
There is one gene for UGTs w/ 5 exons
* exons 2-5 stay the same
* exon 1 can vary based on splicing
* there are 12 possible exon 1s
Describe the UGT1 family gene structure.
There is one gene for UGTs w/ 5 exons
* exons 2-5 stay the same
* exon 1 can vary based on splicing
* there are 12 possible exon 1s
Describe the UGT2 family gene structure.
2 subfamilies: a & b
* 2A1 - only 1 human gene
* 2B4, 7, 10, 15, and 17
Crigler-Nijjar syndrome and Gilbert’s syndrome are due to…
Polymorphisms in UGT1A1 promoter
* affects bilirubin conjugation –> hyperbilirubinemia
Explain how Irinotecan (CPT-11) works in the body as a chemotherapy drug.
CPT-11 is a topoisomerase inhibitor
* CPT-11 activated to SN-38 by carboxylesterases
* SN-38 –> tumor cell death (anticancer effect)
* SN-38 metabolized by UGT1A1 –> then metabolized in the liver –> SN-38G
* goes into the intestine –> excreted
Explain the dose-limiting side effect of Irinotecan.
In people that need a higher dose, they experience toxic effects (bloody diarrhea)
* In the intestine, B-glucuronidase removes the glucuronic acid from SN-38G –> returns to SN-38
* SN-38(active) not stable to b-glucuronidase –> enterhepatic circulation –> gut epithelial cell death (topoisomerase inhibitor) –> dose-limiting bloody diarrhea
Explain a possibility to prevent B-glucuronidase from reactivating SN-38 to produce toxic effects.
Inhibit B-glucuronidase
* try to make inhibitor drug selective for bacterial B-glucuronidase (bind to bacterial loop that isn’t present in humans)