title: Photoconversion of 7-dehydrocholesterol to vitamin D3 in synthetic phospholipid bilayers.
abstract: The incorporation of 7-dehydrocholesterol into synthetic phospholipid bilayers altered the distribution of products after photolysis. In liposomes, the relative amounts of 7-dehydrocholesterol and lumisterol were elevated, and tachysterol was reduced from the levels observed in hexane solution. Z to E isomerization of the previtamin to tachysterol is favored in organic solvents. The inhibition of this process is evidence that an ordered lipid matrix places a new constraint on the conformation of the ring B fission product--one in which the configuration is favorable for a return to a cyclized diene. Further, rate enhancements of up to 15-fold were observed for the thermal isomerization of the previtamin to vitamin D3 in liposomes. The free energies of activation for the reaction at 25 degrees C were reduced by 1.3-1.5 kcal/mol in the bilayer environment compared to that of hexane. As this reaction involves the concerted transfer of a hydrogen via a cyclic intermediate, it provides additional evidence for membrane stabilization of an all-cis conformation of the previtamin. Photoproduct ratios were also studied for 7-dehydrocholesterol adsorbed to a variety of solid supports. That nonspecific interactions of 7-dehydrocholesterol with lipid can influence product formation may have important implications with respect to the mechanism of vitamin D3 biosynthesis.
authors.forename: J K
authors.initials: JK
authors.lastname: Yamamoto
authors.forename: R F
authors.initials: RF
authors.lastname: Borch
mesh_terms: D002151:Calorimetry; D002762:Cholecalciferol; D002775:Cholestadienols; D002851:Chromatography, High Pressure Liquid; D002855:Chromatography, Thin Layer; D003684:Dehydrocholesterols; D007700:Kinetics; D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D010777:Photochemistry; D013329:Structure-Activity Relationship
chemical_list: D002775:Cholestadienols; D003684:Dehydrocholesterols; D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D002762:Cholecalciferol; C016705:7-dehydrocholesterol