HIOMT

HIOMT stands for hydroxyindole-O-methyltransferase, an enzyme that catalyzes the final step of melatonin biosynthesis in many animals, including humans. In practice, HIOMT is commonly identified with acetylserotonin O-methyltransferase (ASMT), and the two terms are often used interchangeably in the literature. The reaction converts N-acetylserotonin to melatonin using S-adenosyl-L-methionine as the methyl donor, making HIOMT a key component of the circadian regulation of melatonin production.

The enzyme is expressed in the pineal gland, where it functions in concert with other enzymes in the melatonin pathway. It is also present in retina and various brain regions, reflecting broader roles in neurochemical signaling. HIOMT activity shows circadian variation, typically peaking during the night when melatonin levels rise. Its regulation involves sympathetic nervous system input and light-dependent signaling that gates the nocturnal synthesis of melatonin. Genetic factors can influence HIOMT/ASMT expression and activity, and multiple ASMT gene variants have been studied for potential associations with sleep traits.

Clinically, alterations in HIOMT/ASMT activity or gene variants have been investigated in relation to sleep and circadian rhythm disorders, as well as seasonal affective disorder. While findings across studies are not entirely consistent, HIOMT remains recognized as the terminal enzyme in melatonin biosynthesis and a contributor to the signaling of darkness to the brain. Research on HIOMT includes biochemical characterization, genetic studies, and investigations of how environmental factors such as light exposure modulate its activity.