PR2155 IC5 med chem (reactive metabolite.)

What are some types of RM?

1. Reactive metabolite funcitonal groups (specific ones)
2. electrophiles (C centred or hetero)
3. radicals (C centred or hetero)

What are the functional groups that are mostly related with RM"?

How do RM cause harm?

1. Inhibit metabolising enzymes (P450) via irreversible metabolism based inhibition
2. Induce adverse drug reaction (hapten formation, immune activation)
3. React with DNA (forming adduct, mutagen)

How do we make RM harmless?

1. Electrophile


1. Glutathione (GSH)
2. Epoxides


1. H2O
2. GSH
3. Induction of protective enzyme


1. NQO1

what are some of the processes that can form RM?

1. P450 oxidation reaction (Phase I)
2. Glucoronidation (Phase II)


1. glucoronosyl transferase
3. Heme peroxidase using H2O2 for oxidation
4. Sulfation (phase II)


1. sulfotransferase
5. Reduction (Phase I)


1. Reductase

Where is P450 ususally found in the liver?

Smooth ER

What are the three components of the P450?

1. P450


1. contains the binding site
2. not specific, can bind to most things lipophilic
2. Cytochrome P450 Reductase (CPR)
3. Membrane matrix

Are oxy ferryl species electron deficient or electron rich?

electron deficient

The O is electron deficient, it is then transferred to the substrate to leave the active site.

What do P450 need in order to carry out oxidation?

Oxygen

NADPH

How will P450 oxidation process possibly generate RM?

1. Reduction


1. the substrate if very reducible, may be reduced instead of the oxygen
2. Radical formation


1. since the reduction occurs as single electron transfers
3. Oxygen rebound


1. potentially resulting in electrophilic metabolite. O2\* might leave.

What are the substrates that P450 can transfer the activated O to?

1. Sp3 carbons
2. sp2 carbons in alkene
3. sp2 carbons in aromatic rings
4. carbon-heteroatom oxidation

What happens during the hydroxylation of sp3 carbon by P450?

Carbon radical is formed

carbon radical stability affects how likely it is formed

What happens during the hydroxylation of sp2 carbon alkene by P450?

Radical addition to the pi bond in the alkene C=C bonds

oxygen rebound occurs and epoxide is formed.

epoxide ring opens and forms an electrophilic carbon

what are the fates of epoxide from alkene?

1. spontaneous reaction with H2O to form diol
2. reaction with GSH
3. forming adducts with macromolecules (bad)

What are the fates of epoxide from aromatic rings?

1. spontaneous reaction with water to form diol
2. GSH
3. forming adducts

What determines the position of the hydroxylation in aromatic ring?

The R group.

If R group is electron donating, it will prefer to go to para

What determines the rate of the hydroxylation in the aromatic ring?

The R group.

The attacking group is looking for electron rich carbon centre. You want the R group to be electron donating to increase the rate.

What are the reactions P450 has on carbon-heteroatoms?

oxidations


1. oxidation of heteroatoms
2. Dealkylation of the side chain attached to heteroatom

What happens in the P450 oxidation of heteroatoms?

Heteroatom has to be soft nucleophile for the reaction to occur

\
O cannot, it is a hard nucleophile

What happens during the dealkylation of side-chains

Dealkylation of ethers.

Hemiacetal is formed and is broken down into alcohol and aldehyde

oxidative N-Dealkylation

product: aldehyde + amine

generally second or thrid degree amines

Amine: Big

Aldehyde: small

Oxidative deamination

Product: small amine + big aldehyde/carbonyl

usually primary, but not necessarily

\

What are the functional groups that can undergo glucoronidation?

COOH

OH

NH2

SH

Comment on the stability of glucoronides.

Acyl glucoronides formed from COOH is unstable

* susceptible to hydrolysis

Ether glucoronides formed from phenolic OH is stable

what kind of reaction does formation of glucuronides involve?

Sn2 reactions

\

What are the fates of O-acyl-glucuronides?

1. Hydrolysis
2. Intramolecular rearrangement
3. transacylation

Which enzyme hydrolyses O-acyl-glucoronides?

Carboxyesterases (CES)

Beta glucoronidases

\
Acyl glucoronides MW>500 undergo enterohepatic cycling

What is the mechanism behind the intramolecular rearrangement?

Nucleophilic OH attacks the electrophilic carbonyl C in ester

What happens in transacylation?

Acyl group is transferred to an incoming nucleophile

lysine NH2

Cysteine SH

Serine OH