A rapid and precise method for the determination of vitamin D3 in rat skin by high-performance liquid chromatography.

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Abstract
To develop the investigations into photobiogenesis of vitamin D3 a rapid and precise method for the determination of the vitamin in rat skin was established by using high-performance liquid chromatography (HPLC). The proposed method included saponification of small pieces of rat skin, extraction of the unsaponifiable matter and application to HPLC using Zorbax SIL (straight-phase) as an adsorbent and 0.5% isopropanol in n-hexane as a mobile phase. The applicable lower limit of the method was 2 ng of vitamin D3/cm2 of subcutaneous tissue-removed skin and it was possible to assay a concentration > 2 ng/cm2. The proposed method was applied to determine the content of vitamin D3 in rat skin obtained from in vivo and in vitro irradiation experiments. In the in vitro experiment the yield of vitamin D3 increased in proportion to the irradiation time. The yield in the in vivo experiment showed a proportional increase similar to the in vitro experiment until 60 min irradiation while a nearly constant value was obtained by irradiation for > 60 min. When the rat skin obtained from the in vitro experiment was irradiated with monochromatic UV rays in the range 260-350 nm the most effective wavelength for the formation of vitamin D3 was confirmed to be 303 nm which differs from the result obtained from the experiment in a test tube (295 nm). The yield of vitamin D3 by irradiation with UV rays below 288 nm was extremely low which differed from the results of a test tube experiment. These differences were thought to be due to the filter effect of the malpighian layer in the epidermis of rat skin.