[2] In relation to this, MAO-A is the main isozyme of MAO involved in serotonin degradation.

[2][3] 5-HIAL can also be converted into small amounts of 5-hydroxytryptophol (5-HTOL; also known as 5-hydroxyindolethanol or 5-HIET) by either aldehyde reductase (ALR/ALDR) or alcohol dehydrogenase (ADH).

[2][4] Use of ethanol (alcohol) can dramatically increase 5-HTOL formation by inhibiting ALDH and enhancing ADH activity.

[2][5] As a result, the ratio of 5-HTOL to 5-HIAA is a sensitive and reliable marker of recent ethanol ingestion and has been suggested for use in clinical and forensic contexts.

[2][5] Besides oxidative deamination by MAO into 5-HIAL, serotonin can also be conjugated by glucuronidation via glucuronyltransferases, conjugated by sulfation via sulfotransferases, acetylated and then methylated into melatonin (N-acetyl-5-methoxytryptamine) (which occurs mainly in the pineal gland), and converted into certain other metabolites like 5-hydroxyindole thiazoladine carboxylic acid (5-HITCA).