Transnitrosation of non-mutagenic N-nitrosoproline forms mutagenic N-nitroso-N-methylurea

N-Nitroso-N-methylurea (NMU) is a potent carcinogen and suspected as a cause of human cancer. In this study, mutagenic NMU was detected by HPLC after the transnitrosation of non-mutagenic N-nitrosoproline (NP) to N-methylurea in the presence of thiourea (TU) under acidic conditions. The structure of NMU was confirmed by comparing (1)H NMR and IR spectra with that of authentic NMU after fractionation by column chromatography. Furthermore, a fraction containing NMU formed by transnitrosation was mutagenic in Salmonella typhimurium TA1535. NMU was formed in the reaction of NP and N-methylurea in the presence of 1,1,3,3-tetramethylthiourea (TTU) or 1,3-dimethylthiourea in place of TU as an accelerator. The reaction rate constants (k) for NMU formation were correlated with their nucleophilicity of sulfur atom in thioureas. The N-methylurea concentration did not affect the NMU formation, whereas the rate of NMU formation correlated linearly with concentrations of NP, TTU and oxonium ion. The observed kinetics suggests a mechanism by which the nitroso group was transferred directly from the protonated NP to the thiourea then to N-methylurea to form NMU. The rate-determining step was the formation of the complex with the protonated NP and thiourea.

Reaction of Vitamin E Compounds withN-Nitrosated Tryptophan Derivatives and Its Analytical Use

We recently showed that nitrosated tryptophan residues may act as endogenous nitric oxide storage compounds. Here, a novel reaction of nitrosotryptophan derivatives is described, in the form of the release of nitric oxide from N-nitrosotryptophan derivatives initiated either by alpha-tocopherol or by its water-soluble form trolox. Alpha-tocopherol and trolox were found to release stoichiometric amounts of nitric oxide from N-acetyl-N-nitrosotryptophan as well as from the nitrosotryptophan residue in albumin. The reaction proceeds both in water and in lipophilic solution and reconstitutes the indole moiety of the tryptophan molecule quantitatively. During this reaction, alpha-tocopherol- and trolox-derived phenoxyl-type radicals were identified as intermediates by ESR spectrometry. The chemical mechanism of the NO-releasing process was established. Since S-nitrosothiols do not react under the applied conditions, it is suggested that the trolox-dependent release of nitric oxide may be utilizable for the detection of N-nitrosotryptophan residues in biological samples. Furthermore, as N-nitrosotryptophan derivatives do not undergo spontaneous decay in lipophilic environments, vitamin E may have the so far unrecognized function of preventing the accumulation of N-nitrosotryptophan residues to toxic concentrations in biological systems.