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. 2022 Mar 4;13:835480. doi: 10.3389/fphar.2022.835480

FIGURE 1.

FIGURE 1

Vitamin D Metabolism: UVB radiation penetrates the skin, converting 7-dehydrocholesterol to pre-vitamin D3, which is rapidly converted to vitamin D3. Vitamin D3 is transported through the circulation to the liver. Dietary vitamin D2 and D3 are transported from the intestine to the liver by chylomicrons (plasma and lymph). In the liver, vitamin D is hydroxylated to 25(OH)D, mediated by CYP2R1 (cytochrome P450 [CYP] enzyme). Once released into the circulation 25(OH)D binds to vitamin D binding protein and is transported to the kidneys and other tissues. In the proximal tubules of the kidney, 1α-hydroxylation (CYP27B1) of 25(OH)D results in the production of the active vitamin calcitriol (1.25(OH)2D). 1.25(OH)2D induces the expression of the enzyme 24-hydroxylase encoded by the CYP24A1 gene which catalyses the conversion of 25(OH)D and 1.25(OH)2D to the inactive 24-hydroxylated products, 24.25(OH)2D and 1,24,25(OH)3D respectively. Reproduced with permission from: Griffin TP, Bell M, Robinson S et al. (2017) Vitamin D and vitamin D deficiency in Ireland - a call to action. UPDATE Endocrinology and Diabetology [Internet]. 2017[37–40 pp.]. Available from: https://www.medicalindependent.ie/100980/update_ endocrinology__diabetology. Accessed 10.01.22).