Three-Dimensional Quantitative Structure-Activity Relationship of ArylalkylamineN-acetyltransferase (AANAT) Inhibitors: A Comparative Molecular Field Analysis

Current melatonin research is essentially based on the finding of new molecular tools, including synthetic or natural agonists and antagonists for the melatonin receptors and synthetic inhibitors of the enzymes involved in its biosynthesis. Indeed, the use of these compounds will improve our understanding of some of the numerous mechanisms of action of melatonin. The present report deals with the establishment and description of a new cell line expressing in a stable manner human arylalkylamine-N-acetyltransferase (AANAT, E.C.2.3.1.87). This new cellular system permits one to check the capacity of newly discovered inhibitors to penetrate the cell and reach their target. Some emphasis is put on inhibitors of the bromoacetyltryptamine family since these precursor compounds form in situ bisubstrate inhibitors with strong affinity for the human enzyme. AANAT is known to undergo complex and rapid regulation by a subtle balance between extremely fast catabolism and protection against it, both due to serine phosphorylation. In the present report, this phosphorylation is shown to occur in vitro after incubation with several kinases (rho-kinase, chk-1, protein kinase A) but not with protein kinase C. Phosphorylation enhances the specific activity of the enzyme by a factor of two to five. This phosphorylation is also shown to occur after treatment of the cell with compounds such as forskolin and rolipram that enhance or protect the intracellular pool of cAMP or the cell-permeable cAMP analogue, dioctanoyl-cAMP. The specificity of the cellular model was assessed using a series of substrates and inhibitors of AANAT already described in the literature, and the characteristics of this cellular system are shown to correspond with those reported for the purified enzyme. This cell line was used to screen libraries of compounds in a living system and led to the discovery of several potent specific and non-toxic AANAT inhibitors.

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New substrate analogues of human serotonin N‐acetyltransferase produce in situ specific and potent inhibitors

Ferry

Ubeaud

Mozo

et al. 2003

European Journal of Biochemistry

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Melatonin is synthesized by an enzymatic pathway, in which arylalkylamine (serotonin) N‐acetyltransferase catalyzes the rate‐limiting step. A previous study [Khalil, E.M., De Angelis, J., Ishii, M. & Cole, P.A. (1999) Proc. Natl Acad. Sci. USA96, 12418–12423] reported the discovery of bromoacetyltryptamine (BAT), a new type of inhibitor of this enzyme. This compound is the precursor of a potent bifunctional inhibitor (analogue of the transition state), capable of interfering with both the substrate and the cosubstrate binding sites. This inhibitor is biosynthesized by the enzyme itself in the presence of free coenzyme A. In the present report, we describe the potency of new N‐halogenoacetyl derivatives leading to a strong in situ inhibition of serotonin N‐acetyltransferase. The new concept behind the mechanism of action of these precursors was studied by following the biosynthesis of the inhibitor from tritiated‐BAT in a living cell. The fate of tritiated‐phenylethylamine (PEA), a natural substrate of the enzyme, in the presence or absence of [3H]BAT was also followed, leading to their incorporation into the reaction product or the inhibitor (N‐acetyl[3H]PEA and coenzyme A‐S[3H]acetyltryptamine, respectively). The biosynthesis of this bifunctional inhibitor derived from BAT was also followed by nuclear magnetic resonance during its catalytic production by the pure enzyme. In a similar manner we studied the production of another inhibitor generated from N‐[2‐(7‐hydroxynaphth‐1‐yl)ethyl]bromoacetamide. New derivatives were also screened for their capacity to inhibit a purified enzyme, in addition to enzyme overexpressed in a cellular model. Some of these compounds proved to be extremely potent, with IC50s of ≈ 30 nm. As these compounds, by definition, closely resemble the natural substrates of arylalkylamine N‐acetyltransferase, we also show that they are potent ligands at the melatonin receptors. Nevertheless, these inhibitors form a series of pharmacological tools that could be used to understand more closely the inhibition of pineal melatonin production in vivo.

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Three-Dimensional Quantitative Structure-Activity Relationship of ArylalkylamineN-acetyltransferase (AANAT) Inhibitors: A Comparative Molecular Field Analysis

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Characterization and regulation of a CHO cell line stably expressing human serotonin N-acetyltransferase (EC 2.3.1.87)

New substrate analogues of human serotonin N‐acetyltransferase produce in situ specific and potent inhibitors

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