Figure 2.

Photobiosynthesis and activation of Vitamin D. 7-dehydrocholesterol in the skin is converted to pre-vitamin D3, upon exposure to sunlight, which contains UVB radiation. Pre-vitamin D3 is converted to vitamin D3 in a heat-dependent process. Vitamin D2 and D3 from the diet are incorporated into chylomicrons and introduced into the circulation. Vitamin D formed in the skin or ingested through diet is stored and released from fat cells. In the serum, vitamin D is circulated while being bound to the vitamin D binding protein, DBP. DBP transports it to the liver where it is converted to 25(OH)D (circulating form) by vitamin D-25-hydroxylase. This form of vitamin D is biologically inactive and is converted in the kidneys to the biologically active form, 1α,25(OH)₂D by 25-hydroxyvitamin D-1α- hydroxylase (1α-OHase). The 1α,25(OH)₂D binds to the membrane vitamin D receptor (mVDR) or the nuclear vitamin D receptor (nVDR) and elicits specific biological responses.