The oxidation of sulfur containing cyclic ketimines The sulfoxide is the main product of S-aminoethyl-cysteine ketimine autoxidation

The oxidation of aminoethylcysteine ketimine dimer by oxygen reactive species

The prominent spontaneous reaction of aminoethylcysteine ketimine in the neutral pH range is the concentration-dependent dimerization (Hermann, 1961). The carboxylated dimer first produced loses the free carboxyl yielding the more stable decarboxylated dimer (named simply the dimer in this note). In the search for a possible biochemical activity of this uncommon tricyclic compound we have assayed whether it could interact with oxygen reactive species (H2O2, O2 (-),(•)OH) thus exhibiting a scavenging effect of possible biomedical interest. The dimer interacts with H2O2 producing compounds detectable by chromatographic procedures. The presence of Fe(2+) stimulates the oxidative reaction by yielding the hydroxyl radical (the Fenton reaction). Using the system xanthine oxidase-xanthine as superoxide producer, the dimer oxidation by O2 (-) has also been documented. Among the oxidation products the presence of taurine and cysteic acid has been established. Identification of remaining oxidation products and investigation of the possible function of the dimer as a biological scavenger of oxygen reactive species are now oncoming.

An insight in the mechanism of the aminoethylcysteine ketimine autoxidation

Oxidation of aminoethylcysteine ketimine (AECK) is followed by the change of 296nm absorbance, by the O2 consumption and by the HPLC analysis of the oxidation products. The oxidation is strongly inhibited by the addition of superoxide dismutase (SOD) but not by hydroxyl radical scavengers or catalase. Addition of EDTA or o-phenanthroline (OPT) favours the oxidation, probably by keeping contaminating metals in solution at the pH studied. Addition of Fe(3+) ions strongly accelerates the oxidation in the presence of EDTA or OPT. AECK reacts stoichiometrically with OPT-Fe(3+) complex producing the Fe(2+) complex which is not reoxidised by bubbling O2. HPLC analyses of the final oxidation products reacting with 2,4-dinitrophenylhydrazine (DNPH) confirm the AECK sulfoxide as the main product of the slow spontaneous oxidation. The detection of other oxidation products when the reaction is speeded up by the addition of the OPT-Fe(3+) complex, suggests that the oxidation takes place essentially on the carbon portion of the AECK molecule in the side of the double bond. On the basis of the results presented here, a scheme of reactions is illustrated which starts with the transfer of one electron from AECK to a contaminating metal ion (possibly Fe(3+)) producing the radical AECK(•) as the initiator of a self propagating reaction. The radical AECK(•) reacting with O2 starts a series of reactions accounting for most of the products detected.

Aminoethylcysteine ketimine decarboxylated dimer detected in normal human urine by gas-liquid chromatography, selected-ion monitoring and mass spectrometry

Aminoethylcysteine ketimine is a biochemical product known to be converted spontaneously in the decarboxylated dimer. Since the ketimine has been detected in a mammalian brain, it was assumed that also the dimer could be present in the mammalian body and eventually excreted in the urine. Using human urine as the biological source, an extract was prepared which, submitted to gas-liquid chromatography, selected-ion monitoring and mass spectrometry, indicated the presence of the dimer.

Identification of new products of S-aminoethylcysteine ketimine autoxidation

In continuation of a previous work (Pecci et al., 1993), dedicated to the detection of the autoxidation products of S-aminoethylcysteine ketimine (AECK), we give here data for the identification of 2,3,6,7-tetrahydro-4H-[1,4]thiazino[2,3-b]thiazine, thiomorpholine-3-one and 5,5', 6,6'-tetrahydro-2,2'-dihydroxy-3,3'-bi-2H-thiazine among the products of AECK autoxidation. Identification has been done on the basis of mass spectrometry and NMR spectral analyses of the isolated products.