We have used isothermal titration calorimetry to study how the membrane lipid composition affects the partitioning of bile salts into membranes. We have compared the partitioning of bile salts into sphingomyelin (SM) and phosphatidylcholine (PC) bilayers. The partitioning of the sodium salts of cholate and deoxycholate, at concentrations below the critical micelle concentration, into egg yolk phosphatidylcholine (EPC), hydrogenated EPC (HEPC), and egg yolk sphingomyelin (ESM) membranes was studied at 25 degreesC. Deoxycholate (0.3 mM) partitioned into ESM membranes with a K value of 2200 +/- 100 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 ESM but not the EPC membranes. The partition enthalpy for deoxycholate was 13.2 +/- 2.8 and 10.8 +/- 1.3 kJ mol(-1) in ESM and EPC membranes, respectively. The partitioning of cholate at 0.3 mM into ESM and EPC membranes did not 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 membranes (K of 410 +/- 10 M-1, DeltaH of 9 +/- 0.5 kJ mol(-1)). These results show that bile salt partitioning into membranes is influenced both by the phospholipid type (SM versus PC) and by the acyl chain composition (unsaturated versus saturated).