<prompt> 25 hydroxylation of vitamin D occursSkinLiverKidneySpleen  <response>: Synthesis of 25-OH-D3 in Liver CalcidiolVitamin D2 andor 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.Coenzymecofactors required are: MgNADPH, 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 