cholecalciferol and betadex

To examine the effect of microenvironments on previtamin D3<==>vitamin D3 isomerization, we have conducted kinetic studies of the reaction in an aqueous solution of beta-cyclodextrin. Our results showed that at 5 degrees C, the forward (k1) and reverse (k2) rate constants for previtamin D3<==>vitamin D3 isomerization were increased by more than 40 and 600 times, respectively, compared with those in n-hexane (k1, 8.65 x 10(-6) versus 1.76 x 10(-7) s-1; k2, 8.48 x 10(-6) versus 1.40 x 10(-8) s-1), the fastest rate of this isomerization ever reported at this temperature. Thermodynamic studies revealed that the equilibrium constant of the reaction was significantly reduced by more than 12-fold when compared to that in n-hexane at 5 degrees C, and the percentage of vitamin D3 at equilibrium was increased as the temperature was increased in beta-cyclodextrin. When complexed with beta-cyclodextrin, the previtamin D3<==>vitamin D3 isomerization became endothermic (delta H zero = 13.05 kJ mol-1) in contrast to being exothermic in other media. We propose that thermodynamically unfavorable cZc conformers of previtamin D3 are stabilized by beta-cyclodextrin, and thus the rate of the isomerization is increased. This conformation-controlled process may play an important role in the modulation of previtamin D3<==>vitamin D3 endocrine system in vivo such as in the sea urchin.

Topics: Animals; beta-Cyclodextrins; Catalysis; Cholecalciferol; Cyclodextrins; Isomerism; Kinetics; Sea Urchins; Temperature; Thermodynamics

The interaction of beta-cyclodextrin (beta-CD) with four bile acids, cholic, taurocholic, chenodeoxycholic and lithocholic, was demonstrated by proton-NMR spectroscopy. Lithocholic and chenodeoxycholic acids exhibit a stronger affinity for beta-CD than cholic and taurocholic acids. The affinity of bile acids for beta-CD increases in relation to their hydrophobicity. The competition between these bile acids and the lipophilic vitamins A and D3 in the formation of beta-CD inclusion complexes was studied. These vitamins compete with cholic and taurocholic acids whereas they do not with lithocholic and chenodeoxycholic acids. In the latter case all of the beta-CD present was consumed by the bile acids with the vitamins remaining free in the medium. The affinity of vitamins A and D3 for beta-CD is lower than that of the bile acids. Therefore, when lithocholic or chenodeoxycholic acids are present, the formation of beta-CD inclusion complexes with the vitamins does not occur. The results of this study suggest that depletion of lipophilic vitamins will not occur upon ingestion of beta-CD, thus providing further support for the safety and suitability of beta-CD as an ingredient in foods and orally administered drugs.

Topics: beta-Cyclodextrins; Bile Acids and Salts; Binding, Competitive; Chenodeoxycholic Acid; Cholecalciferol; Cholic Acid; Cholic Acids; Cyclodextrins; Humans; Lithocholic Acid; Magnetic Resonance Spectroscopy; Taurocholic Acid; Vitamin A