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• Glucuronide and sulfate conjugates also can undergo hydrolytic cleavage by β-glucuronidase and sulfatase enzymes

  
• Phase I (Functionalization) reactions

  Do not always produce hydrophilic or pharmacologically inactive metabolites and nontoxic

  
• Phase II (Conjugation) reactions

  Can convert Phase I metabolites to more polar and water-soluble products
• Phase II conjugation reactions

  • Glucuronic acid conjugation
  • Sulfate conjugation
  • Amino acid conjugation (glycine, glutamine)
  • Glutathione (GSH) conjugation
  • Acetylation
  • Methylation

  
• Conjugating groups


  Are activated initially in the form of a coenzyme before transfer or attachment to the accepting substrate by the appropriate transferase enzyme
• Glucuronic acid conjugation (Glucuronidation)


  Major phase II metabolic pathway for drugs, xenobiotics and endogenous compounds

  
• Formation of β-glucuronides


  1. Synthesis of activated coenzyme, uridine-5-diphospho-α-D-glucuronic acid (UDPGA)

  2. Transfer of glucuronyl group from UDPGA to substrate, catalyzed by UDP-glucuronyltransferases

  
• All glucuronide conjugates have the β-configuration or β-linkage at C-1

  
• Drugs undergoing O-glucuronidation


  • At phenolic and alcoholic hydroxyl groups
  • At carboxylic groups

  
• Drugs undergoing N-glucuronidation


  • Aromatic & aliphatic amines
  • Amides
  • Sulfonamides

  
• Drugs undergoing S-glucuronidation


  • Compounds with free SH group

  
• Drugs undergoing C-glucuronidation


  • Relatively rare pathway
• Sulfate conjugation (Sulfation)


  Less frequent than glucuronidation, mainly for phenols, alcohols, aromatic amines, N-hydroxyls
• Formation of sulfate conjugates

  1. Activation of inorganic sulfate (SO4) to 3'-phosphoadenosine-5'-phosphosulfate (PAPS)

  2. Transfer of sulfate group from PAPS to substrate, catalyzed by sulfotransferases
• Sulfate conjugation generally leads to water-soluble and inactive metabolites
• Drugs undergoing sulfation


  • α-methyldopa, salbutamol, terbutaline

  
• Glucuronidation and sulfation of phenols

  Glucuronidation is frequently a competing reaction to sulfation

  
• Neonates and young children have decreased glucuronidating capacity but well-developed sulfation, making sulfation the major route for acetaminophen conjugation

  
• Sulfate conjugation of some N-OH containing compounds can produce reactive toxic molecules

  
• Amino acid conjugation


  Less frequent than glucuronidation, involves conjugation of carboxyl group (particularly aromatic/arylalkyl acids) with glycine or glutamine
• Formation of amino acid conjugates

  1. Carboxylic acid substrate is activated with ATP and CoA to form acyl-CoA

  2. Acyl-CoA is conjugated with glycine or glutamine by N-acyltransferase enzymes

  
• Drugs undergoing amino acid conjugation

  • Brompheniramine, benzoic acid, salicylic acid

  
• Glutathione (GSH) conjugation


  Important pathway for detoxifying chemically reactive electrophilic compounds by forming S-substituted GSH adducts

  
• Xenobiotics conjugated with GSH usually are not excreted directly, but undergo further metabolism to mercapturic acid conjugates
• Brompheniramine


  • Oxidized to a propionic acid metabolite that is conjugated with glycine

  
• Benzoic acid


  • Undergoes glycine conjugation

  
• Salicylic acid


  • Undergoes glycine conjugation
• Glutathione (GSH) conjugation


  An important pathway for detoxifying chemically reactive electrophilic compounds
• Reactive electrophilic species


  • Exert toxicity by combining covalently with nucleophilic groups present in vital cellular proteins and nucleic acids
• GSH


  Protects vital cellular constituents against chemically reactive species by its nucleophilic SH group
• GSH

  A tripeptide (γ-glutamyl-cysteinyl glycine)
• Xenobiotics conjugated with GSH


  1. Usually are not excreted as such, but undergo further biotransformation to give S-substituted N-acetylcysteine products called mercapturic acids

  2. Involves enzymatic cleavage of two amino acids (glutamic acid and glycine) from the initially formed GSH adduct

  3. Subsequent N-acetylation of the remaining S-substituted cysteine residue

  
• GSH S-transferases


  Enzymes that catalyze conjugation of substrates with GSH
• Unlike other conjugative phase II reactions, GSH conjugation does not require the initial formation of an activated coenzyme or substrate</b>
• Substrates susceptible to GSH conjugation

  • Must be sufficiently electrophilic
• Formation of GSH conjugates of electrophilic xenobiotics or metabolites (E)

  Conversion to mercapturic acids

  
• Arene oxides and epoxides are intermediary products formed from CYP oxidation of aromatic compounds (arenes) and olefins, respectively, and are "neutralized" or detoxified by GSH S-conjugation

  
• Acetylation


  Metabolic pathway to terminate the biological activity and detoxification of drugs and xenobiotics

  
• Functional groups undergoing acetylation

  • Primary aromatic amines (ArNH2)
  • Sulfonamides (H2NC6H4SO2NHR)
  • Hydrazines (-NHNH2)
  • Hydrazides (-CONHNH2)
  • Primary aliphatic amines (RNH2; PhNH2)

  
• Amide derivatives formed from acetylation of these amino functionalities are generally inactive and nontoxic