Interaction of Bile Salts with Model Membranes Mimicking the Gastrointestinal Epithelium: A Study by Isothermal Titration Calorimetry

© 2015 American Chemical Society. Bile salts (BS) are biosurfactants synthesized in the liver and secreted into the intestinal lumen where they solubilize cholesterol and other hydrophobic compounds facilitating their gastrointestinal absorption. Partition of BS toward biomembranes is an important step in both processes. Depending on the loading of the secreted BS micelles with endogeneous cholesterol and on the amount of cholesterol from diet, this may lead to the excretion or absorption of cholesterol, from cholesterol-saturated membranes in the liver or to gastrointestinal membranes, respectively. The partition of BS toward the gastrointestinal membranes may also affect the barrier properties of those membranes affecting the permeability for hydrophobic and amphiphilic compounds. Two important parameters in the interaction of the distinct BS with biomembranes are their partition coefficient and the rate of diffusion through the membrane. Altogether, they allow the calculation of BS local concentrations in the membrane as well as their asymmetry in both membrane leaflets. The local concentration and, most importantly, its asymmetric distribution in the bilayer are a measure of induced membrane perturbation, which is expected to significantly affect its properties as a cholesterol donor and hydrophobic barrier. In this work we have characterized the partition of several BS, nonconjugated and conjugated with glycine, to large unilamellar vesicles (LUVs) in the liquid-disordered phase and with liquid-ordered/liquid-disordered phase coexistence, using isothermal titration calorimetry (ITC). The partition into the liquid-disordered bilayer was characterized by large partition coefficients and favored by enthalpy, while association with the more ordered membrane was weak and driven only by the hydrophobic effect. The trihydroxy BS partitions less efficiently toward the membranes but shows faster translocation rates, in agreement with a membrane protective effect of those BS. The rate of translocation through the more ordered membrane was faster, indicating accumulation of BS at specific locations in this membrane.Peer Reviewe

Synthesis and characterization of a lipidic alpha amino acid: Solubility and interaction with serum albumin and lipid bilayers

10 pags, 7 figs, 1 tab. -- Supporting Information is available at Publisher's webThe lipidic α-amino acid with 11 carbons in the alkyl lateral chain (α-aminotridecanoic acid) was synthesized via multicomponent hydroformylation/Strecker reaction, which is a greener synthetic approach to promote this transformation relative to previously described methods. Its solubility and aggregation behavior in aqueous solutions was characterized, as well as the interaction with lipid bilayers. Lipidic amino acids are very promising molecules in the development of prodrugs with increased bioavailability due to the presence of the two polar functional groups and nonpolar alkyl chain. They are also biocompatible surfactants that may be used in the food and pharmaceutical industry. In this work we have conjugated the lipidic amino acid with a fluorescent polar group (7-nitrobenz-2-oxa-1,3-diazol- 4-yl), to mimic drug conjugates, and its association with serum proteins and lipid bilayers was characterized. The results obtained indicate that conjugates of polar molecules with lipidic α-amino acid, via covalent attachment to the amine group, have a relatively high solubility in aqueous solutions due to their negative global charge. They bind to serum albumin with intermediate affinity and show a very high partition coefficient into lipid bilayers in the liquid-disordered state. The attachment of the polar group to the lipidic amino acid increased strongly the aqueous solubility of the amphiphile, although the partition coefficient into lipid membranes was not significantly reduced. Conjugation of polar drugs with lipidic amino acids is therefore an efficient approach to increase their affinity for biomembranes. © 2013 American Chemical Society.We thank Fundação para a Ciência e Tecnologia (COMPETE - Programa Operacional Factores de Competitividade), QREN/FEDER (PTDC/QUI-QUI/112913/2009 and PTDC/QUI/64565), and for the integrated action E-07/12. A.A., A.P., H.F., and F.C.-G. thank FCT for their Grants (SFRH/BD/73190/2010, SFRH/BPD/72126/2010, SFRH/BD/65375/2009 and SFRH/BD/40778/2007, respectively) and the Spanish Ministry of Science and Innovation (BFU2010-19451 and PRI-AIBPT-2011-1025)

Essential Role for the CRAC Activation Domain in Store-dependent Oligomerization of STIM1

Oligomerization of the ER Ca2+ sensor, STIM1, is the key triggering step in the activation of store-operated Ca channels. We show that the cytosolic CRAC activation domain, previously known only for its role in binding to and activating Orai1 channels, is also required for stable STIM1 oligomerization