Vitamin D exists in two forms -
(vitamin D3) and ergocalciferol (vitamin D2). Cholecalciferol
is produced from cholesterol, and ergocalciferol from a plant- or microbe-derived precursor, ergosterol. Vitamin D can be obtained via the diet, or as a result of the action of ultraviolet light on
in the skin.
By hydroxylation, it is converted into
in the liver, and
in the kidneys. It is interesting to note how the addition of these hydroxyl groups adds to the solubility of these compounds.
Calcitriol is the active form of vitamin D, and functions as a hormone promoting the uptake of calcium ions from the gut into the blood. It is thus important in the formation of bones, and in its absence rickets may develop.