The formation of 25-hydroxycholecalciferol takes place in the-

In response to ultraviolet radiation of the skin, a photochemical cleavage results in the formation of vitamin D from 7-dehydrocholesterol. Vitamin D enters the circulation, whether absorbed from the intestine or synthesized cutaneously, bound to vitamin D-binding protein, an a-globulin synthesized in the liver. Vitamin D is subsequently 25-hydroxylated in the liver by cytochrome P450-like enzymes in the mitochondria and microsomes. 25-hydroxyvitamin D (25D), is the major circulating and storage form of vitamin D. The second hydroxylation, required for the formation of the mature hormone, occurs in the kidney. Reference : page 2463-64 Harrison's Principles of Internal Medicine 19th edition