EXAM 5 - Cofactors for Phase 2 Metabolism 1

Question 1

During glucuronidation, where is the electrophilic site? What occurs there?

Answer 1

Electrophilic site on glucuronic acid
* The substrate (drug) binds to the electrophilic site

Question 2

During sulfation/sulfonation, where is the electrophilic site?

Answer 2

Electrophilic site on sulfate group

Question 3

During acetylation, where is the electrophilic site?

Answer 3

Electropilic site on acetate group

Question 4

During methylation, where is the electrophilic site?

Answer 4

Electrophilic site on methyl group of sulfonium ion.

Question 5

During amino acid conjugation, where are the nucleophilic sites?

Answer 5

Nucleophilic site on NH2 of each amino acid.

Question 6

During glutathione conjugation, where is the nucleophilic site located?

Answer 6

Nucleophilic site on SH of cysteine
* entire molecule of glutathione gets added to the substrate

Question 7

How many UGT gene families are there?

Answer 7

2 gene families: UGT1 and UGT2
* each with many UGTs
* all microsomal (embedded in the ER membrane)

Question 8

Describe the process of UDP-GA synthesis (UGT cofactor).

Answer 8

1. starts as glucose
2. enzyme uses UTP to take off 2 phosphate groups
3. oxidized to glucuronic acid –> UDP-GA
4. transfered across the ER to the lumen of the ER

Question 9

Explain the process of glucuronidation of a nucleophilic drug (-OR) by UGT.

Answer 9

• 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)

Question 10

Describe the product of OH conjugation by glucuronic acid.

Answer 10

Glucuronidation of alcohol forms ether
* ETHER: R-O-R’
* oxygen on the alcohol groups attacks the electrophilic site of glucuronic acid

Question 11

Describe the product of acid conjugation by glucuronic acid.

Answer 11

Product: esters
* nucleophilic oxygen on the acid is going to attack the UDP-GA
* ESTER =

Question 12

Explain what groups substrates must have in order to get glucuronidated.

Answer 12

• OH
• COOH
• NH2
• SH
• C–C

Question 13

Can glucuronidation form toxic products?

Answer 13

Yes.

Question 14

Describe what a glucuronide is.

Answer 14

• The product of glucuronidation
• a substrate that is linked to glucuronic acid

Question 15

Describe what a glucuronide is.

Answer 15

• The product of glucuronidation
• a substrate that is linked to glucuronic acid

Question 16

Describe when glucuronides end up in the urine versus the bile.

Answer 16

Glucuronide MW..
MW > ~400 –> bile
MW < ~300 –> urine

Question 17

Describe the UGT1 family gene structure.

Answer 17

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

Question 18

Describe the UGT1 family gene structure.

Answer 18

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

Question 19

Describe the UGT2 family gene structure.

Answer 19

2 subfamilies: a & b
* 2A1 - only 1 human gene
* 2B4, 7, 10, 15, and 17

Question 20

Crigler-Nijjar syndrome and Gilbert’s syndrome are due to…

Answer 20

Polymorphisms in UGT1A1 promoter
* affects bilirubin conjugation –> hyperbilirubinemia

Question 21

Explain how Irinotecan (CPT-11) works in the body as a chemotherapy drug.

Answer 21

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

Question 22

Explain the dose-limiting side effect of Irinotecan.

Answer 22

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

Question 23

Explain a possibility to prevent B-glucuronidase from reactivating SN-38 to produce toxic effects.

Answer 23

Inhibit B-glucuronidase
* try to make inhibitor drug selective for bacterial B-glucuronidase (bind to bacterial loop that isn’t present in humans)