Mechanistic studies on the alkyltransferase activity of serotonin N-acetyltransferase

Background: Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT) catalyzes the first, rate-limiting step in the biosynthesis of the circadian hormone melatonin (5-methoxy-N-acetyltryptamine) from serotonin. Our recent discovery that, in addition to catalyzing the acetyl transfer from acetyl-coenzyme A (acetyl-CoASH) to serotonin, AANAT is also a robust catalyst for the alkyl transfer reaction between CoASH and N-bromoacetyltryptamine has not only opened up a new way to develop cell-permeable AANAT acetyltransferase inhibitors that are valuable in vivo tools in helping elucidate melatonin's (patho)physiological roles, but has also raised a question - how does AANAT accelerate the alkyl transfer reaction? In this study, mechanistic aspects of the AANAT-catalyzed alkyl transfer reaction were explored by employing CoASH and a series of N-haloacetyltryptamines that were also evaluated for their AANAT acetyltransferase inhibitory activities. Results: Investigation of various N-haloacetyltryptamine analogs showed a similar leaving group effect on the enzymatic and non-enzymatic reaction rates. Steady-state kinetic analyses demonstrated that AANAT alkyltransferase obeys a sequential, ternary complex mechanism, with random substrate binding. Rate versus pH profiles revealed the catalytic importance of an ionizable group with pK_a∼7. All those N-haloacetyltryptamines that serve as substrates of AANAT alkyltransferase are also potent (low micromolar) in vitro inhibitors against AANAT acetyltransferase activity. In particular, N-chloroacetyltryptamine was also shown to be a potent inhibitor of intracellular melatonin production in a pineal cell culture assay. Conclusions: This is the first detailed investigation of the alkyltransferase activity associated with an acetyltransferase. Our results indicate that AANAT does not accelerate the alkyl transfer reaction by simple approximation effect as previously proposed for the similar alkyl transfer reaction catalyzed by other acyltransferases. This study has general implications for developing novel inhibitors by taking advantage of the promiscuous alkyltransferase activity associated with several acyltransferases.