Methylation is a common but generally minor pathway of xenobiotic biotransformation.
Unlike most other conjugative reactions, methylation often does not dramatically alter the
solubility of substrates and results either in inactive or active compounds. Methylation
reactions are primarily involved in the metabolism of small endogenous compounds such as
neurotransmitters but also play a role in the metabolism of macromolecules for example
nucleic acids and in the biotransformation of certain drugs. A large number of both
endogenous and exogenous compounds can undergo N– (Fig. 6a), O– (Fig. 6b), S– (Fig. 6c)
and arsenic–methylation during their metabolism (Feng et al., 2010). The co–factor required
to form methyl conjugates is S–adenosylmethionine (SAM), which is primarily formed by
the condensation of ATP and L–methionine