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
BIG SHOT: P450 SIDEKICK: IRON-OXO COMPLEX O=FeV 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 3A4 Most 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!) 2D6 Genetic 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 2C19 Genetic polymorphism (some Caucasians, lots of Asians), not as worrisome as 2D6 2E1 Alcohol inducible form, likes small substrates
All the different ways P450 will eat this drug P450 SIDEKICK: IRON-OXO COMPLEX FeV ALDEHYDE AMINE Cl- 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 ALDEHYDE ARENE OXIDE Penultimate carbon Last resort Epoxide hydrolase CANNOT work here, arene oxide falls apart quickly PHENOL e- donating: ortho/para e- withdrawing: meta Halogens: ortho/para EPOXIDE TRANS DIOL
The others QUINONE REDUCTASE ACID AMINE ESTERASE Amidase is much slower than esterase Hydrolysis Equilibrium reaction AMIDASE & ALDEHYDE DEHYDROGENASE AMIDE ACID ESTER 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 1 QUINONE HYDROQUINONE ALDEHYDE Best substrate is the 1 alcohol, since equilibrium lies on the left 2 alcohols can t be oxidized to acid ACID 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
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 (NH 2 ), and sulfhydryl (-SH) groups Nutritional factors affect phase II metabolism
1. Glycoside Conjugation -- conjuguation with glucuronic acid Enzyme: UDP glucuronosyl transferase Requires activated form of glucose UDPG SN2 reaction (drug is nucleophile) that forms the β linkage Sugar added at phenols, alcohols, aromatic amines, acids 2. Sulfate Conjugation -- formation of sulfate esters Enzyme: sulfotransferase Requires activated form of sulfur PAPS Drug is the nucleophile Sulfonic acid added at phenols, aromatic amine, alcohols 3. Glutathione Conjugation removal of electrophiles Enzyme: glutathione S-transferase GST GS conjugates usually converted to mercapturic acid form Eliminates epoxides, RX, α,β-unsaturated carbonyls, quinones, quinone imines, quinone methides, etc. 4. Amino Acid Conjugation Enzyme: acyl synthetase transacetylase Usually conjugates with carboxylic acids, which needs to be activated to CoA thioester first Mainly 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.