92 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 3.5 Figure 6. Chromatogram of a sample from receptor cell at 48 hours in vitamin E skin permeation study. sample from the receptor solution after 48 hours of skin permeation study. Total radioactivity was separated into three peaks. The peaks with retention times of 13 and 9 minutes were identified as ot-tocopherol and ot-tocopherol quinone, respectively. Considering our separation conditions, as shown in Table I, we can conclude that vitamin E was metabolized to a more hydrophilic metabolite, which may be an ot-to- copherol quinol as described in Figure 7 (13). The disappearance of peaks from time to time in the HPLC chromatogram, as observed in Figure 1, implies that there is some interaction between the dermis of the skin and vitamin E in receptor solution. This may be caused by back diffusion or further metabolism taking place by the enzyme leached from the dermis (14). METABOLISM OF VITAMIN E IN RECEPTOR SOLUTION WITHOUT CONTACTING THE SKIN To confirm that vitamin E is metabolized in the skin, a stability study of vitamin E in the receptor solution was performed. In Figure 8, the chromatograms clearly show that vitamin E was stable in the receptor solution for up to 24 hours. Vitamin E in contact with the skin showed very significant metabolic degradation, as shown in Figure 6. Although vitamin E undergoes some degradation in the receptor solution (first order degradation constant, K = 0.026 hr-1, data not shown) (15), there is no other peak except for vitamin E observed in UV analysis. In the liquid scintillation counter, most of the radioactivity was recovered as ot-tocopherol. This result clearly indicates that the major metabolism of vitamin E takes place in the skin and not in the receptor solution. In 1974, Shiratori observed the high and prolonged retention of radioactivity in the skin, comparing other tissue after intravenous injection of tritium-labeled vitamin E, and he suggested that skin may have an important storage, excretion, and metabolism function of vitamin E even though he couldn't provide the evidence (16). The present

VITAMIN E METABOLISM 93 CH 3 Fe scorc etc. Acid, F AgNO3 CH 3 16H33 ¸ H3 CH 3 3 METASTABLE EPOXIDE IRREVERSIBLY ON STANDING CH 3 O•C16H33 H3C ]' •' •)•CH3 CH3 a-TOCOPHERYL QUINONE 2H Zn CH3-COOIt CH 3 HO ••C16tt33 •H3C •[ •r•CH 3 CH 3 QUINOL Figure 7. Schematic diagram of the formation of o•-tocopherol quinone and quinol from o•-tocopherol (from ref. 13). experimental results shown in Figures 6-8 indicate that vitamin E follows a very complicated metabolic pathway. At this stage of research, however, it can be concluded that the skin is capable of vitamin E metabolism and that vitamin E is metabolized to its more hydrophilic matabolites, which may be more easily excreted from the body.