Solubilization and Biochemical Characterization of the Melatonin Deacetylase from Xenopus laevis Retina

Abstract

Melatonin deacetylase, an enzyme activity recently discovered in the Xenopus laevis retina, regulates local melatonin levels. The deacetylase occurs in retina, retinal pigment epithelium, and skin, all sites of melatonin action, and is widely distributed among vertebrates. We have solubilized the enzyme from Xenopus retina and pigment epithelium using nonionic detergents, and have developed a specific enzyme assay. We have characterized the enzyme and now report that the deacetylase is relatively specific for melatonin and is inhibited by the melatonin precursor N-acetylserotonin and the product of the deacetylase, 5-methoxytryptamine. Inhibition of deacetylase activity by eserine (physostigmine) suggests a relationship between deacetylase and cholinesterase activities. However, among a variety of cholinesterase inhibitors tested, only eserine inhibits the deacetylase. Furthermore, eserine is much less potent as an inhibitor of the deacetylase than the cholinesterases, and purified cholinesterases failed to deacetylate melatonin. We also show that melatonin deacetylase and aryl acylamidase (an enzyme related to cholinesterases) activities are differentially extractable from Xenopus ocular tissues, and that they exhibit different pH optima and inhibition profiles. Our results provide an initial characterization of the Xenopus retinal melatonin deacetylase, and indicate that deacetylase activity is distinct from cholinesterase and aryl acylamidase activities.

Serotoninergic and melatoninergic systems are fully expressed in human skin

“…6B) or that newly formed melatonin is degraded rapidly. In frog skin, there is an extensive metabolism of melatonin with a rate of degradation even higher than in the brain or retina (41, 42). Further characterization of those pathways is the subject of our future research that will combine the use of a wide range of metabolic inhibitors with mass spectrometry‐derived methods.…”

Melatonin, hormone of darkness and more – occurrence, control mechanisms, actions and bioactive metabolites

“…The light-dependent decrease of retinal melatonin is caused by dopamine via D 1 or D 4 receptors [18]. Differences in metabolism exist, too, because retinal melatonin is largely degraded by deacetylation to 5-methoxytryptamine (5-MT) [12,19], a compound which can be further converted to 5-methoxytryptophol (5-ML) or 5-methoxyindoleacetic acid (5-MIAA) [8,10,20]. In the retina, deacetylation to 5-MT can be catalyzed by a specific melatonin deacetylase, but in other organs or organisms by less specific aryl acylamidases (AAAs) or eserine-sensitive acetylcholinesterase [20].…”

Melatonin Metabolism in the Central Nervous System

“…2). The enzyme identified in the Xenopus retina was later named melatonin deacetylase [32][33][34]. It displays high substrate specificity for melatonin and is clearly distinct from different, less specific AAAs.…”

“…In general, melatonin deacetylating enzymes can be classified as aryl acylamidases (AAAs). Although the conversion of melatonin to 5-MT by these enzymes seems to be important in organisms different from animals, such as dinoflagellates [37,45] and yeast [129], the quantitative significance of this pathway has only become apparent in the retinas of some amphibians, reptiles and fish [19,32,34]. In other retinas in which AA-NAT is strongly expressed, but only small amounts of melatonin are detected, melatonin deacetylation may be considered, too, as long as the low melatonin levels are not attributable to poor Omethylation.…”