تفاعلاتُ المرحلةِ الثانيةِ: تفاعلاتُ المثيلةِ

Methylation involves two sequential steps. First, a methyltransferase enzyme catalyzes the transfer of a methyl group from S-adenosylmethionine (SAM) to the substrate, producing S-adenosylhomocysteine (SAH) as a byproduct. Second, SAH undergoes further metabolism into homocysteine, which can be recycled back to methionine through remethylation, completing the cycle.

Methyltransferases such as catechol-O-methyltransferase (COMT) and thiopurine methyltransferase (TPMT) catalyze methylation reactions. These enzymes target functional groups including hydroxyl, amine, thiol, and carboxylic acid groups on drug substrates. The methylation process modifies these groups to facilitate drug metabolism and elimination.

Methylation is essentially the reverse of phase I demethylation, involving the addition of a methyl group rather than its removal. However, methylation operates through a phase II-type reaction mechanism, making it a conjugation-style biotransformation. This distinction reflects methylation's role in drug inactivation and elimination despite its mechanistic similarity to phase I processes.

Methylated products typically exhibit altered pharmacological activity compared to parent drugs. While they generally show equal or enhanced activity in some cases, they often display decreased affinity for target receptors or enzymes. Notably, methylated metabolites are not necessarily more water-soluble than their parent drugs, distinguishing methylation from other phase II conjugation reactions.

Codeine is methylated by methyltransferase enzymes, which transfer a methyl group from SAM to hydroxyl groups on the codeine molecule. This methylation converts codeine into morphine, a more potent analgesic. This transformation exemplifies how methylation can enhance a drug's pharmacological activity and demonstrates the clinical significance of phase II methylation reactions.

Methylation is a significant reaction in both the biosynthesis and inactivation of endogenous amines. For example, dopamine undergoes methylation to produce epinephrine, a hormone critical to the body's stress response. This demonstrates that methylation extends beyond drug metabolism to regulate important physiological processes involving neurotransmitters and hormones.

Although methylation reverses phase I demethylation mechanistically, it employs a phase II-type reaction mechanism characteristic of conjugation reactions. Methylation uses a coenzyme (SAM) and produces a byproduct (SAH), similar to other phase II processes. This classification reflects methylation's role in drug detoxification and elimination, transforming lipophilic drugs into metabolites for excretion.