Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Vitamins and Trace Elements: Their Role in Enzyme Function
Vitamin D as a Hormone Precursor

Vitamin D deficiency leads to impaired phosphorus-calcium METABOLISM and disrupts normal Bone Formation. As a result, children may develop Rickets, characterized by such typical symptoms as lower limb deformities and chest wall malformations (Pigeon chest). Vitamin D₃, or cholecalciferol (Fig. 10-21), is normally synthesized in the Skin of humans and animals from an inactive precursor, 7-dehydrocholesterol, through a series of reactions initiated by ultraviolet radiation from sunlight. Vitamin D₃ is also abundant in fish Liver oil. Another common form of vitamin D, vitamin D₂ or ergocalciferol, is produced industrially by UV irradiation of Yeast ergosterol. As long as an individual receives adequate exposure to sunlight, supplemental vitamin D is not required.

In recent years, the biochemical role of vitamin D has been extensively studied. Vitamin D₃ itself lacks biological activity; rather, it serves as a precursor to 1,25-dihydroxycholecalciferol (Fig. 10-22). Vitamin D₃ undergoes a two-step hydroxylation process—first in The Liver and subsequently in the Kidneys. 1,25-dihydroxycholecalciferol is synthesized in the kidneys and transported to other Organs and Tissues, primarily the Small Intestine and bones, where it regulates Ca²⁺ and phosphorus Homeostasis. Due to this characteristic mode of action, 1,25-dihydroxycholecalciferol can be classified as a hormone, since a hormone is defined by definition as a chemical messenger synthesized in one organ that regulates the biological activity of another tissue.