Page 3 Phase II Conjugation Conjugation reactions involve the addition of molecules naturally present in the body to the drug molecule. The drug may have undergone a phase I reaction. This is the main conjugation reaction in the body. This occurs in the liver.
Natural substrates are bilirubin and thyroxine.
Aliphatic alcohols and phenols are commonly conjugated with glucuronide. Thus hydroxylated metabolites can also be conjugated. Generally the resultant metabolite is more water soluble, and certainly less lipid soluble. Less drug is reabsorbed from the kidney. Occasionally the metabolite is less water soluble.