A study of the photobiogenesis of cholecalciferol in vivo and the constraint on its 25-hydroxylation in rat

Vitamin D binding protein is required to utilize skin-generated vitamin D

Vitamin D is produced in the skin following exposure to sunlight. Ultraviolet (UV) B (UVB, 280-310 nm) results in isomerization of 7-dehydrocholesterol to previtamin D that spontaneously isomerizes to vitamin D. This pool of skin-derived vitamin D is the major source of vitamin D for animals. However, the mechanisms by which it becomes available remain undefined. It has been assumed that cutaneous vitamin D is transported into the circulation by vitamin D binding protein (DBP), but experimental evidence is lacking. To determine whether cutaneous vitamin D is transported by DBP, we utilized DBP^-/- mice that were made vitamin D-deficient. These animals lack measurable 25(OH)D in blood and are hypocalcemic. As controls, DBP^+/+ animals were vitamin D depleted and made equally hypocalcemic. UV irradiation of DBP^+/+ animals restored serum calcium and serum 25(OH)D while the same treatment of DBP^-/- animals failed to show either a serum calcium or 25(OH)D response despite having normal vitamin D production in skin. Intravenous injection of small amounts of recombinant DBP to the vitamin D-deficient DBP^-/- mice restored the response to UV light. These results demonstrate a requirement for DBP to utilize cutaneously produced vitamin D.

Effect of u.v. rays on 7-dehydrocholesterol content in rat skin

Long-term exposure of rat to ultraviolet irradiation affected the cholesterol metabolism in the skin. The 7-dehydrocholesterol content was increased to 1.6 times of non-irradiated rat. However, the cholesterol content was not altered. Ultraviolet-induced precholecalciferol and cholecalciferol bound with an activator protein of delta 5,7 sterol-delta 7-reductase. This observation suggested that precholecalciferol and cholecalciferol bound with an activator protein of delta 5,7 sterol-delta 7-reductase. This observation suggested that precholecalciferol and cholecalciferol inhibit the conversion of 7-dehydrocholesterol to cholesterol and accumulate 7-dehydrocholesterol. The action of ultraviolet rays on the cholesterol metabolism in the skin may be explained as the action of ultraviolet-induced precholecalciferol and cholecalciferol.

The relationship of adequate and excessive intake of vitamin D to health and disease

Vitamin D is required for intestinal calcium and phosphorus absorption, kidney functions, bone synthesis, and possible vital functions in several other target tissues. Active forms of vitamin D are generated by hydroxylations, first in the liver and second in the kidney. The requirement for vitamin D is normally met by its synthesis in the skin. Inadequate skin exposure to sunlight, dark-pigmented skin, and a northern geographical latitude make some individuals susceptible to a deficiency. In the United States, various foods are fortified with vitamin D to ensure that such deficiencies do not occur. As a result, most individuals consume and synthesize more vitamin D than they require. Vitamin D is a toxic compound, and excessive amounts can cause soft-tissue calcification. We have suggested a mechanism by which this calcification might occur. There is a narrow leeway between the amount required and that initiating tissue damage. As most individuals appear to be at risk of obtaining too much vitamin D rather than too little, we suggest that fortification of foods with vitamin D should be curtailed, preferably abolished, that excessive fortification of animal foods be reduced to the level required, and that the use of dietary supplements be restricted. Populations at risk could be monitored closely and counseled to prevent vitamin D deficiency.