Abstract
It is now firmly established that vitamin D3 (cholecalciferol) must first be converted to more active forms by a series of hydroxylations before exerting its characteristic effects on calcium metabolism. The first hydroxylation takes place predominantly in the liver where cholecalciferol is converted to 25-hydroxycholecal-ciferol (25 OH D3), the major circulating form of the vitamin (l). A second hydroxylation takes place only in the kidney; 25 OH D3 is hydroxylated by a mitochondrial enzyme system either in position I to form 1,25 dihydroxycholecalciferol (1,25(OH)2D3) (2), the most active metabolite of vitamin D3 known. An alternative step hydroxylates 25 OH D3 in position 24 to produce 24,25 dihydroxycholecalciferol (24,25 (OH)2D3), a less active metabolite (3).
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