Vitamin A in human skin: I. detection and identification of retinoids in normal epidermis

In an attempt to identify vitamin A and derivatives (retinoids) specimens of breast skin epidermis (0.5 g) were homogenized, freeze-dried and extracted with chloroform/methanol. The evaporated extract was partitioned repeatedly between petroleum ether and a mixture of ethanol and pH-adjusted water. This yielded 3 fractions of partially purified retinoids. High-pressure liquid chromatography (HPLC) of these fractions revealed the presence of the following retinoids given in order to their abundance in the epidermis: retinyl acyl esters, retinol, 3-dehydroretinyl acyl esters and retinoic acid. Small amounts of other retinoids may also be present. In order to obtain quantitative data it was essential to add internal retinoid standards and to completely hydrolyze the skin in KOH-ethanol before extraction. The retinoids were deconjugated by this procedure but, with the exception of retinaldehyde, were otherwise unchanged. The recoveries of the endogenous retinoids at HPLC were identical to those of the internal standards. The technique was reproducible and could be applied to the analysis of nanograms of retinol and dehydroretinol in small (10-30 mg) skin specimens. The amounts of acidic retinoids were usually below the detection limit of the method (less than 10 ng/g) but the approach may be useful at the higher levels attained during retinoid therapy.