Vitamins A and D as Prohormones

In addition to the coenzyme function of retinal in vision another vitamin A derivative, retinoic acid, is an important hormone with effects on differentiation of cells and tissues. It acts to control transcription of the genetic messages in DNA by binding to specific protein receptors that in turn bind to specific nucleotide sequences of the DNA. The retinoid receptor proteins are a member of the steroid hormone receptor family. Also related to this family are receptors for hydroxylated derivatives of vitamin D.

Vitamin D can be viewed as a prohormone which arises by the action of ultraviolet light in the two-step process pictured in Fig. 20. Irradiation of 7-dehydrocholesterol in the skin can provide adequate amounts of vitamin D₃ (cholecalciferol or calciol). The closely related vitamin D₂ (ergocalciferol) arises from irradiation of the plant sterol ergosterol. This form of the vitamin has been widely used in fortification of milk. However, the natural vitamin D₃ is more active in preventing rickets. The term vitamin D₁ was dropped when it was found to be a mixture of D₂ and D₃. The principal function of vitamin D is in the control of calcium metabolism. This control is exerted by polar, hydroxylated compounds of which the most important is 1α, 12-dihydroxyvitamin D₃ (calcitriol). This hormone is distributed to all parts of the body. In cells of the intestinal lining it promotes uptake of calcium ions. It promotes reabsorption of both calcium and phosphate ions in the kidney tubules and increases blood calcium and depositon of calcium ions in bone.