J Biol Chem, Vol. 274, Issue 7, 4174-4179, February 12, 1999

A Liposomal Model That Mimics the Cutaneous Production of Vitamin D₃
STUDIES OF THE MECHANISM OF THE MEMBRANE-ENHANCED THERMAL ISOMERIZATION OF PREVITAMIN D₃ TO VITAMIN D₃

Xiao Quan Tian and Michael F. Holick^§

From the Vitamin D, Skin, and Bone Research Laboratory, Endocrinology Nutrition and Diabetes Section, ^§ Departments of Medicine and  Physiology, Boston University Medical Center, Boston, Massachusetts 02118

We reported previously that the rate of previtamin D₃ (preD₃)  vitamin D₃ isomerization was enhanced by about 10 times in the skin compared with that in organic solvents. To elucidate the mechanism by which the rate of this reaction is enhanced in the skin, we developed a liposomal model that mimicked the enhanced isomerization of preD₃ to vitamin D₃ that was described in human skin. Using this model we studied the effect of changing the polarity of preD₃ as well as changing the chain length and the degree of saturation of liposomal phospholipids on the kinetics of preD₃  vitamin D₃ isomerization. We found that a decrease in the hydrophilic interaction of the preD₃ with liposomal phospholipids by an esterification of the 3-hydroxy of preD₃ (previtamin D₃-3-acetate) reduced the rate of the isomerization by 67%. The addition of a hydroxyl on C-25 of the hydrophobic side chain (25-hydroxyprevitamin D₃), which decreased the hydrophobic interaction of preD₃ with the phospholipids, reduced the rate by 87%. In contrast, in an isotropic n-hexane solution, there was little difference among the rates of the conversion of preD₃, its 3-acetate, and 25-hydroxy derivatives to their corresponding vitamin D₃ compounds. We also determined rate constants (k) of preD₃  vitamin D₃ isomerization in liposomes containing phosphatidylcholines with different carbon chain lengths. The rates of the reaction were found to be enhanced as the number of carbons (C_n) in the hydrocarbon chain of the phospholipids increased from 10 to 18. In conclusion, these results support our hypothesis that amphipathic interactions between preD₃ and membrane phospholipids stabilize preD₃ in its "cholesterol like" cZc-conformer, the only conformer of preD₃ that can convert to vitamin D₃. The stronger these interactions were, the more preD₃ was likely in its cZc conformation at any moment and the faster was the rate of its conversion to vitamin D₃.