25 hydroxylation of vitamin D occurs

Synthesis of 25-OH-D3 in Liver (Calcidiol)Vitamin D2 and/or D3 binds to specific D binding protein and is transpoed to liver.It undergoes hydroxylation at 25 position, by the enzyme 25-hydroxylase, in the endoplasmic reticulum of the mitochondria of liver cells.Coenzyme/cofactors required are: *Mg++NADPH, and *Molecular O2 A cytoplasmic factor is also required, the exact nature not known. Two enzymes, an NADPH-dependant cytochrome P450 reductase and a cytochrome P450 are involved also.25-OH-D3 (calcidiol) is the major storage form of vitamin D in liver and found in appreciable amount in circulation. The blood level of 25-OH-D3 exes feedback inhibition on the enzyme 25-hydroxylase. Reference: Chatterjea Textbook of Biochemistry pg no. 168