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

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 x 75 microm) with 0.05 M NaH2PO4 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 x 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 x 10(-5) 0.96+/-0.26 x 10(-5) and 1.04+/-0.30 x 10(-5) M, respectively, (mean+/-SEM, n=8). The results obtained by HPCE correlated well with data obtained by HPLC. Correlation coefficients of r2=0.981, r2=0.814 and r2=0.956 were found for histidine, trans- and cis-urocanic acid, respectively.