Methylation is a common but generally minor pathway of xenobiotic
biotransformation. Methylation differs from most other
phase II reactions because it generally decreases the watersolubility
of xenobiotics and masks functional groups that might
otherwise be conjugated by other phase II enzymes. One exception
to this rule is the N-methylation of pyridine-containing xenobiotics,
such as nicotine, which produces quaternary ammonium
ions that are water-soluble and readily excreted.