1-alpha hydroxylation in vitamin d metabolism takes place in

(C) (Kidney) (681 - Ganong 23rd)Vitamin Do Vitamin D is the major vitamin involved in the absorption of calciumo Beside dietary intake, cutaneous synthesis is the other and more important source of vitamin D (cholecalciferol) in the body. The keratinocytes of stratum corneum of the epidermis of skinQ contain 7- dehydrocholesterol which is converted vitamin D3 (Cholecalciferol) by the action of sunlight (UV light),o This vitamin D3 (cholecalciferol) is converted to calcidiol (calcifediol or 25 hydroxy cholecalciferol or 25- hydroxyl vitamin D3) BY 25 hydroxylase in the liverQo Calcidiol is the major circulating form of vitamin D3o Calcidiol is then converted to Calcitriol (1-25 dihydroxy-cholecalciferol or 1-25 dihydroxy vitamin D3) BY 1-a hydroxylation in kidneyQ.o The final hydroxylation in kidney is rate limiting stepQo Calcitriol is the active form of vitamin DQo Dietary vitamin D is absorbed in ileum 0 and requires bile salts.o When Ca+2 level is high , little calcitriol is produced, and the kidney produce the relatively inactive metabolite 24, 25 dihydroxycholecalciferol insteadQ7-Dehydrocholesterol