• Polar bodies are either introduced or unmasked, which results in more polar metabolites•Phase I reactions can lead either to activation or inactivation of the drug (i.e. therapeutic effects or toxicity)•Oxidation/reduction, hydroxylation, hydrolysis, cyclization, and decyclization reactions•Most common is oxidation by cytochrome P450 monooxygenase•A common phase I oxidation involves conversion of a C-H bond to a C-OH

P450SIDEKICK:IRON-OXO COMPLEXO=FeV

2C19Genetic polymorphism (some Caucasians, lots of Asians), not as worrisome as 2D6

3A4Most common, mediates 50% of phase I reactions, non-specific, doesn’t care what its substrate is, some overlap, grapefruit juice is an inhibitor (don’t drink and drug!)

2E1Alcohol inducible form, likes small substrates

2D6Genetic polymorphism (Caucasians are commonly poor metabolizers, sorry!), metabolizes drugs with aliphatic amines (Arg) and a negative side chain that is charged at physiological pH, most anti-depressants, lots of OTC drugs are 2D6 inhibitors

•ER membrane bound•Active site has an iron protoporphyrin IX hydrophobic pocket that likes greasy compounds like drugs•Two steps: abstracts H atom and then rebounds because radical desperately wants to be stable and uncharged, Fe3 is resting state and substrate is oxidized•Side kick: iron oxo complex O=FeV does all the work

BIG SHOT:

+

3°>2°>1°Mechanism same as all the other P450’s, but the hydroxylated product (carbanolamine) is unstable and falls apart to a 2°amine and aldehyde/ketone

+

+Cl-

TRANS DIOL•Penultimate carbon•Last resort

EPOXIDEALCOHOL

ALDEHYDE

ALCOHOL ALCOHOL

ALDEHYDE

ALCOHOL AMINE

ARENE OXIDE

PHENOL•Epoxide hydrolase CANNOT work here, arene oxide falls apart quickly

e- donating: ortho/parae- withdrawing: metaHalogens: ortho/para

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ALCOHOL & ALDEHYDE DEHYDROGENASE

AMIDASE

ESTERASE

QUINONE REDUCTASE +

+

ACID

ACID

AMINE

ALCOHOL

•A favorite for prodrugs•Hydrolysis•Equilibrium reaction•Charged acid at physiological pH and alcohol both↑hydrophilicy making it easier to excrete and also sets up two points for phase II metabolism

•Amidase is much slower than esterase•Hydrolysis•Equilibrium reaction

1 °ALCOHOL

ACID

ALDEHYDE

•Best substrate is the 1° alcohol, since equilibrium lies on the left•2° alcohols can’t be oxidized to acid

QUINONE

HYDROQUINONE

•Purpose: runs around reducing quinones•2 e- reduction, “redox cycling”•Quinones hydroquinones w/o formation of reactive species•Chemopreventive enzyme•Highly inducbile by weak electrophiles•Detox, but phase I

ESTER

AMIDE

•Functional grouped masked by the addition of conjugates•Conjugates: glucuronic acid, sulfonates, glutathione, or amino acids (glycine)•Sites of conjugation reactions carboxyl (-COOH), hydroxyl (-OH), amino (NH2), and sulfhydryl (-SH) groups•Nutritional factors affect phase II metabolism

1. Glycoside Conjugation -- conjuguation with glucuronic acidEnzyme: UDP glucuronosyl transferaseRequires activated form of glucose UDPGSN2 reaction (drug is nucleophile) that forms the β linkageSugar added at phenols, alcohols, aromatic amines, acids

2. Sulfate Conjugation -- formation of sulfate estersEnzyme: sulfotransferaseRequires activated form of sulfur PAPSDrug is the nucleophileSulfonic acid added at phenols, aromatic amine, alcohols

3. Glutathione Conjugation – removal of electrophilesEnzyme: glutathione S-transferase “GST”GS conjugates usually converted to mercapturic acid formEliminates epoxides, RX, α,β-unsaturated carbonyls, quinones, quinone imines, quinone methides, etc.

4. Amino Acid ConjugationEnzyme: acyl synthetase transacetylaseUsually conjugates with carboxylic acids, which needs to be activated to CoA thioester firstMainly glycine as the amino acid

In a normal healthy person, small amounts of quinone imine are formed, which is highly reactive and conjugates with GSH to be readily excreted, thereby detoxifying it. However, if one night you decide to consume large amounts of Miller lites AND pop a few Tylenols to relieve your headache, you’ve made a big mistake you’ll regret. Alcohol induces CYP2E1, causing it to oxidize acetaminophen more rapidly, meaning more quinone imine is formed, and more GSH is used up to detoxify it, which eventually gets depleted, like every good thing in life. Although the GSH is gone, your reactive nucleophile (the quinone imine) is still around, looking to wreak havoc with whatever other nucleophiles it can find. It starts reacting with cysteine residues on proteins, causing covalent modification of proteins (P-SH), which leads to hepatotoxicity and a very unhappy you.