WAS THE FORMATION OF 1,25-DIHYDROXYVITAMIN-D3 INITIALLY A CATABOLIC PATHWAY

Following solar ultraviolet radiation, epidermal 7-dehydrocholesterol is converted to previtamin D3, which then undergoes a thermal isomeriz ation into vitamin D3. The metabolism of vitamin D3, which is usually considered as an inactive compound, gives rise to the active hormone 1 ,25-dihydroxyvitamin D3, following two hydroxylation steps occurring i n liver and kidney. Here, we propose that this anabolic pathway can al so be interpreted as a catabolic one leading to the degradation of the photoproducts of 7-dehydrocholesterol, for which a specific biologica l role in the skin is proposed.