Biosynthesis of 3-dehydroretinol (vitamin A2) from all-trans-retinol (vitamin A1) in human epidermis.

Since the origin of 3-dehydroretinol in epidermis is unknown, we have investigated the possible conversion of all-trans-retinol to 3-dehydroretinol by organ cultured keratome slices (0.3 mm) of human breast skin. [3H]Retinol bound to retinol-binding protein (RBP) was incubated for 24 h with the tissue sample, which was then extracted and analyzed by high-performance liquid chromatography. Radioactive material that comigrated with authentic 3-dehydroretinol was purified to homogeneity. The identity of this material was established by treatment with HCl which resulted in a typical formation of the anhydro form of 3-dehydroretinol. 3-[3H]Dehydroretinol could not be detected in the incubation medium and was not found in the skin when the sample was heat-inactivated before incubation. The tissue production of 3-[3H]dehydroretinol from [3H]retinol continued when the tracer was removed from the medium, attaining a maximum value of 25% of the retinol value at 50 h. It is suggested that epidermal 3-dehydroretinol in vivo originates from serum retinol delivered to the keratinocytes by RBP.