A thermodynamic study of bile salt interactions with phosphatidylcholine and sphingomyelin unilamellar vesicles

We have used isothermal titration calorimetry to study how the membrane lip id composition affects the partitioning of bile salts into membranes. We ha ve compared the partitioning of bile salts into sphingomyelin (SM) and phos phatidylcholine (PC) bilayers. The partitioning of the sodium salts of chol ate and deoxycholate, at concentrations below the critical micelle concentr ation, into egg yolk phosphatidylcholine (EPC), hydrogenated EPC (HEPC), an d egg yolk sphingomyelin (ESM) membranes was studied at 25 degreesC. Deoxyc holate (0.3 mM) partitioned into ESM membranes with a K value of 2200 +/- 1 00 M-1, whereas the K was 520 +/- 30 M-1 with EPC membranes under identical conditions. At concentrations above 0.6 mM, deoxycholate solubilized the E SM but not the EPC membranes. The partition enthalpy for deoxycholate was 1 3.2 +/- 2.8 and 10.8 +/- 1.3 kJ mol(-1) in ESM and EPC membranes, respectiv ely. The partitioning of cholate at 0.3 mM into ESM and EPC membranes did n ot give measurable heats. At 1 mM, cholate partitioning into EPC membranes was characterized by a K of 123 +/- 1 M-1 and a DeltaH of 11 +/- 0.5 kJ mol (-1). Deoxycholate (at 1 mM) partitioned into hydrogenated EPC (K of 275 +/ - 5 M-1, DeltaH of 7 +/- 0.5 kJ mol(-1)) a lower extent than into EPC membr anes (K of 410 +/- 10 M-1, DeltaH of 9 +/- 0.5 kJ mol(-1)). These results s how that bile salt partitioning into membranes is influenced both by the ph ospholipid type (SM versus PC) and by the acyl chain composition (unsaturat ed versus saturated).