Spread Monomolecular Films of Monohydroxy Bile Acids and Their Salts: Influence of Hydroxyl Position, Bulk pH, and Association with Phosphatidylcholine
详细信息 查看全文
- 作者：Monika R. Leonard ; Melissa A. Bogle ; Martin C. Carey ; and Joanne M. Donovan
- 刊名：Biochemistry
- 出版年：2000
- 出版时间：December 26, 2000
- 年：2000
- 卷：39
- 期：51
- 页码：16064 - 16074
- 全文大小：142K
- 年卷期：v.39,no.51(December 26, 2000)
- ISSN：1520-4995
文摘
Undissociated dihydroxy bile acids, alone or with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), lie with their long axes parallel to aqueous-lipid interfaces [Fahey, D. A., Carey,M. C., and Donovan, J. M. (1995) Biochemistry 34, 10886-10897]. To test the generality of this orientation,we used an automated Langmuir-Pockels surface balance to examine pressure-molecular area isothermsand dipole moments of insoluble monohydroxy bile acids and their salts, which are sparingly solublebecause of their presumed high Krafft points. We studied lithocholic acid (LCA) (the natural 3
-OHisomer), glycolithocholic acid (GLCA) (its glycine conjugate), and the semisynthetic isomers, 7-OH-and 12-OH-cholanoic acids with and without POPC, at pH values ranging from 2 to 12. Monolayercollapse pressures increased sigmoidally with ionization, giving apparent pK values of 7.0-8.5 and implyinga stronger affinity of the bile salt anions for the interface. At monolayer collapse, the molecular area ofLCA was ~85 Å2 independent of pH, consistent with the steroid nucleus lying flat. In contrast, the interfacialarea of 7-OH-cholanoic acid decreased from ~80 Å2 at pH 2 to ~40 Å2 above pH 9, consistent with amore vertical orientation and approximating 12-OH-cholanoic acid, which exhibited a molecular area of~45 Å2 at all pH values. All monohydroxy bile acids condensed POPC monolayers more effectively atlow than at high (ionized) pH. We conclude that the 3-OH group is crucial for anchoring bile acids andtheir salts to the aqueous interface, with all monohydroxy species condensing phospholipid membranesregardless of ionization state.
-OHisomer), glycolithocholic acid (GLCA) (its glycine conjugate), and the semisynthetic isomers, 7-OH-and 12-OH-cholanoic acids with and without POPC, at pH values ranging from 2 to 12. Monolayercollapse pressures increased sigmoidally with ionization, giving apparent pK values of 7.0-8.5 and implyinga stronger affinity of the bile salt anions for the interface. At monolayer collapse, the molecular area ofLCA was ~85 Å2 independent of pH, consistent with the steroid nucleus lying flat. In contrast, the interfacialarea of 7-OH-cholanoic acid decreased from ~80 Å2 at pH 2 to ~40 Å2 above pH 9, consistent with amore vertical orientation and approximating 12-OH-cholanoic acid, which exhibited a molecular area of~45 Å2 at all pH values. All monohydroxy bile acids condensed POPC monolayers more effectively atlow than at high (ionized) pH. We conclude that the 3-OH group is crucial for anchoring bile acids andtheir salts to the aqueous interface, with all monohydroxy species condensing phospholipid membranesregardless of ionization state.