Enzyme studies on the esterification of vitamin D in rat tissues.

1. The mechanism of vitamin D esterification in the rat was studied with liver, small-intestinal mucosa, pancreatic juice and blood plasma as enzyme sources and [1-(3)H]cholecalciferol, [U-(14)C]ergocalciferol and [4-(14)C]cholesterol as substrates. 2. No esterification of vitamin D could be detected with liver preparations nor with homogenates or acetone-dried powder extracts of intestinal mucosa. 3. Pancreatic juice esterified [1-(3)H]cholecalciferol with oleic acid, and specificity studies indicated that a cholesterol-esterifying enzyme was using vitamin D as substrate. 4. Plasma cholesterol-esterifying enzyme also esterified vitamin D. 5. The specificity of the esterification reaction is discussed in relation to (a) the molecular structure of the substrates and (b) their availability, in a micellar solution, to the enzyme. 6. It is concluded that cholesterol-esterifying enzymes esterify vitamin D in vivo during absorption from the small intestine and while it is transported in blood.