Identification of epidermal L-tryptophan and its oxidation products by in vivo FT-Raman Spectroscopy further supports oxidative stress in patients with vitiligo

In the past, non-invasive in vivo FT-Raman spectroscopy has been used to detect H2O2-mediated oxidation of methionine to methionine sulfoxide and methionine sulfone, as well as cysteine to cysteic acid, in the sequence of proteins in the epidermis of patients with vitiligo. L-tryptophan (Trp) is another potential target for this oxidation. Owing to the presence of 10(-3) M epidermal albumin which contains one Trp residue, it was tempting to follow the oxidation of this amino acid. Using in vivo and in vitro FT-Raman spectroscopy, we show for the first time that epidermal Trp is oxidised in patients with vitiligo, yielding 5-OH-Trp at 930 cm(-1) and other oxidation products (i.e. N-formyl kynurenine and kynurenine) from indole ring oxidation peaking at 1050 cm(-1). On the basis of detailed in vitro results, we could conclude that 5-OH-Trp as well as formyl kynurenine and kynurenine are formed via H2O2-mediated Fenton chemistry. These results once again bring out the strength of non-invasive in vivo FT-Raman Spectroscopy in dermatology to follow the effect of oxidative stress in the skin of patients with vitiligo. Copyright (c) 2008 John Wiley & Sons, Ltd.