<prompt> Which of the following is rate limiting step?25 hydroxylation of vitamin D24 hydroxylation of vitamin D1 hydroxylation of vitamin DNone  <response>: Vitamin D is a prohormone. The cholecalciferol is first transpoed to liver, where hydroxylation at 25th position occurs, to form 25-hydroxycholecalciferol 25-HCC. The hepatic 25-hydroxylase is a microsomal monooxygenase. It requires cytochrome P-450 and NADPH Fig. 33.9. 25-HCC is the major storage form. ii. In plasma, 25-HCC is bound to vitamin D binding protein VDBP, an alpha-2 globulin.iii. In the kidney, it is fuher hydroxylated at the 1st position. The 1-alpha-hydroxylase is located in mitochondria of proximal convoluted tubules. It requires cytochrome P-450, NADPH and ferredoxin an iron-sulfur protein. Thus 1,25-dihydroxycholecalciferol DHCC is generated. Since it contains three hydroxyl groups at 1, 3 and 25 positions, it is also called Calcitriol Fig. 33.9. The calcitriol thus formed is the active form of the vitamin; it is a hormone See Box 33.1.Ref: DM Vasudevan - Textbook of Biochemistry, 6th edition, page no: 385 