Light emission from tryptophan oxidation by hypobromous acid

Abstract

The emission of ultraweak light from cells is a phenomenon associated with the oxidation of biomolecules by reactive oxygen species. The indole moiety present in tryptophan, serotonin and melatonin is frequently associated with the emission of light during the oxidation of these metabolites. This study presents results for hypobromous acid (HOBr) oxidation of tryptophan as a putative endogenous source of ultraweak light emission. We found that chemiluminescence elicited by the oxidation of tryptophan by HOBr was significantly higher than by hypochlorous acid (HOCl). This difference was related to secondary oxidation reactions, which were more intense using HOBr. The products identified during oxidation by HOCl, but depleted by using HOBr, were N-formylkynurenine, kynurenine, 1,2,3,3a,8,8a-hexahydro-3a-hydroxypyrrolo[2,3-b]-indole-2-carboxylic acid, oxindolylalanine and dioxindolylalanine. The emission of light is dependent on the free α-amino group of tryptophan, and hence, the indole of serotonin and melatonin, although efficiently oxidized, did not produce chemiluminescence. The emission of light was even greater using taurine monobromamine and dibromamine as the oxidant compared to HOBr. A mechanism based on bromine radical intermediates is suggested for the higher efficiency in light emission. Altogether, the experimental evidence described in the present study indicates that the oxidation of free tryptophan or tryptophan residues in proteins is an important source of ultraweak cellular emission of light. This light emission is increased in the presence of taurine, an amino acid present in large amounts in leukocytes, where this putative source of ultraweak light emission is even more relevant.

Taurine Bromamine: Reactivity of an Endogenous and Exogenous Anti-Inflammatory and Antimicrobial Amino Acid Derivative

“…This principle seems to be applicable in our studies, because, in contrast to HOBr and HOCl, Tau-NHBr was able to oxidize tryptophan but not tyrosine. Following this idea, we suspect that our recent proposal for the selectivity of Tau-NHBr [ 40 ] and Tau-NBr 2 [ 41 ] for tryptophan residues in albumin and lysozyme could be reinforced by these new findings. Thus, aiming to advance this proposal, dansylglycine was used to probe the depletion of tryptophan in human serum albumin (HSA).…”

Overlooked Contribution of the Indole Moiety to the Formation of Haloacetonitrile Disinfection Byproducts

“…In the “aniline pathway”, the pyrrole ring in the indole moiety opens after reaction with halogen, forming 4-(2-formamidophenyl)-4-oxobutanoic acid (II.1) and then 4-(2-aminophenyl)-4-oxobutanoic acid (II.2), a substituted aniline. These steps are analogous to tryptophan transforming to N -formyl-kynurenine and then kynurenine by catabolism, sunlight/UV photolysis, as well as oxidation by free chlorine, hydrogen peroxide, , and chlorine dioxide . The subsequent steps in the pathway (II.3–II.4) involve extensive halogen substitution on the aniline aromatic ring, which eventually breaks the ring aromaticity by forcing a double bond to an external N (II.3 top) or O (II.3 bottom) and facilitates ring cleavage at the C–C bond β to the CN or α to the CO.…”