Vitamin D3 is chemically closely allied to the classical steroid hormones. Technically it is a secosteroid; that is, due to the breakage of the carbon-9, carbon-10 carbon–carbon bond, the B ring is opened up so that only the A, C, and D rings are intact. As discussed in Chapter 9, there are several families of vitamin D steroids (i.e., vitamin D2, vitamin D3, vitamin D4) that depend on the structure of the secosteroid side chain. When the side chain is identical to that of cholesterol, then it is the naturally occurring form and belongs to the vitamin D3 family.
Vitamin D3 can be produced photochemically by sunlight from a precursor, 7-dehydrocholesterol, present in the skin or alternatiavely obtained dietarily. Vitamin D3 is known to be metabolized into a family of over 30 daughter metabolites.
The hormonally active steroid that produces the spectrum of biological responses attributable to vita min D3 is 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3]. The major form of vitamin D3 present in blood is 25-hydroxyvitamin D3 [25(OH)D3]. The enzymes in the liver that convert vitamin D3 into 25(OH)D3 are localized in both the mitochondria and the endoplasmic reticulum, while the enzymes in the kidney that catalyze the conversion of 25(OH)D3 into the steroid hormone 1,25(OH)2D3 or 24,25(OH)2D3 are located only in the mitochondria of the proximal kidney tubule. All three enzymes (the 25-, 1α- and 24- hydroxyl) contain cytochrome P450-type mixed- function oxidases. See Chapter 9 for more details.
