Which of the following is rate limiting step?

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