The specificity of the enzymes helps explain many drug interactions. Genetic differences among patients may alter response. Glucuronidation, the most common phase II reaction, is the only one that occurs in the liver microsomal enzyme system. Glucuronides are secreted in bile and eliminated in urine. Chloramphenicol, meprobamate, and morphine are metabolized this way.
Acetylation is the primary metabolic pathway for sulfonamides.
Hydralazine, isoniazid, and procainamide are also acetylated. The sulfate esters formed are polar and readily excreted in urine. Drugs that form sulfate conjugates include acetaminophen, estradiol, methyldopa, minoxidil, and thyroxine. Methylation is a major metabolic pathway for inactivation of some catecholamines. Niacinamide and thiouracil are also methylated.