First insights into regiospecific transnitrosation reactions between tryptophan derivatives: melatonin as an effective target

Abstract

Melatonin, a derivative of the essential amino acid tryptophan, has been portrayed as a hormone, a tissue factor, an autocoid, a paracoid, and a vitamin with antioxidative capabilities. In the present study a novel reaction which cannot be attributed to any of these suggested features, i.e. the transfer of the nitroso-function from N-nitrosotryptophan derivatives to melatonin, is unequivocally demonstrated. In the lipophilic buffer dimethylsulfoxide reaction of N-acetyl-N-nitrosotryptophan (NANT) with melatonin was very slow (k = 1.5 x 10(-6)/m/s), but reversible as shown by 15N-NMR spectrometry. These measurements demonstrated also that the thermodynamical equilibrium lies on the side of N-nitrosomelatonin (NOMela). Quantum-chemical calculations performed with the third-generation density functional B97-2 additionally predicted that this is also the case in an aqueous environment. In fact, reaction of melatonin with either NANT or N-nitrosotryptophan located at the endothelin-1 fragment 16-21 yielded NOMela with a rate constant of 1.7 +/- 0.5/m/s as shown by capillary zone electrophoresis. Interestingly, the known reactive nitrogen oxide species scavenger, piperazine, did not inhibit the NANT-dependent nitrosation of melatonin, thus very strongly indicating a direct transnitrosation reaction. All of these capabilities are known from the reaction of S-nitrosothiols with thiolate anions and are believed to be highly important in the transport and targeting of nitric oxide.

Detection of N‐nitrosomelatonin and other N‐nitrosotryptophan derivatives by transnitrosation of APF and DAF‐2

“…Since N ‐nitrosotryptophan derivatives can transnitrosate other indole compounds [16] as well as aromatic amines [14] it is strongly expected that fluorescent dyes can be generated via the following reactions: …”

N‐Nitrosomelatonin: synthesis, chemical properties, potential prodrug

“…In addition, from reaction of NOMela with thiols or N ‐terminal blocked tryptophan derivatives, the corresponding nitroso compounds, i.e. S ‐nitrosothiols and N ‐nitroso ‐N ‐terminal blocked tryptophan derivatives, respectively, could be identified by N‐15 NMR spectrometry [11, 12, 36], thereby clearly supporting a direct reaction as outlined in Reaction 3b.…”

“…35% and 65%, respectively), as proven by N‐15 nuclear magnetic resonance (NMR) spectrometry [6, 10–12]. The detailed protocol for the preparation of NOMela is given in the Journal of Pineal Research [11, 13]. As the procedure was still further improved in order to optimize the yield, we briefly describe these modifications in the following: the preparation is now completely performed in a cold (4°C) room and the reaction vessel is with the aid of an aluminium foil permanently protected against incident light.…”

“…A true transnitrosation reaction is a reversible one [9], and in fact, the transnitrosation reaction between N ‐nitroso‐ N ‐terminal blocked tryptophan derivatives and melatonin is a reversible reaction in nonaqueous solution [11]. In aqueous solution, however, the equilibrium of Reaction 2 is nearly completely shifted to the right side [11]. By this means and with respect of the current knowledge, NOMela would be endogenously generated only via Reaction 2.…”

“…The simplest transnitrosation, i.e. the exchange of the NO + function from NOMela to a N ‐terminal blocked tryptophan derivative, proceeds only in nonaqueous solution [11]. The hydrolysis reaction (YH=H 2 O) of N ‐nitrosotryptophan derivatives at pH 1–7 is a proton‐catalyzed process (Reaction 3a) [32] and the same should be true for NOMela.…”