Determination of histidine and urocanic acid isomers in the human skin by high-performance capillary electrophoresis

Abstract

Histidine was baseline separated from histamine, 1-methylhistamine and cis- and trans-urocanic acid using high-performance capillary electrophoresis (HPCE) on a fused-silica column (50 cm×75 μm) with 0.05 M NaH 2PO 4 buffer, pH 5.0, and 12 kV. The detection limit of histidine, trans- and cis-urocanic acid was 10 −6 M at a wavelength of 214 nm. The detection limit of the urocanic acid isomers was slightly enhanced to 5·10 −7 M at 267 nm. The transformation of the trans-urocanic acid standard in vitro into the cis-isomer was dependent on the time of exposure and the energy of the light source. UVB light induced a significantly faster conversion than UVA light. The HPCE method was used for the characterization and measurement of histidine and urocanic acid in human skin eluates. The concentrations of histidine, trans- or cis-urocanic acid in ethanol washes from the skin of healthy, non-allergic volunteers were 2.22±0.40·10 −5, 0.96±0.26·10 −5 and 1.04±0.30·10 −5 M, respectively, (mean±SEM, n=8). The results obtained by HPCE correlated well with data obtained by HPLC. Correlation coefficients of r 2=0.981, r 2=0.814 and r 2=0.956 were found for histidine, trans- and cis-urocanic acid, respectively.