Interaction between the 35 kDa apolipoprotein of pulmonary surfactant and saturated phosphatidylcholines. Effects of temperature.

We studied the interaction between the 35 kDa apolipoprotein of canine pulmonary surfactant (SP 35) and five saturated phosphatidylcholines: distearoyl (DSPC), diheptadecanoyl (DHPC), dipalmitoyl (DPPC), dimyristoyl (DMPC), and dilauroyl (DLPC); and two monoenoic unsaturated phosphatidylcholines: dioleoyl (DOPC) and dielaidyl (DEPC), using temperatures at which all of the phospholipids except DOPC were in both the gel and liquid-crystalline states. The experiments were carried out in a buffer without Ca2+. The amount of apolipoprotein which was bound by both small unilamellar and multilayered vesicles of these lipids decreased as the temperature was increased. Moreover, near the temperatures of the phase transitions of all lipids except DLPC, there was an abrupt and marked reduction in binding of protein, in that over a 3-4 degree change in temperature there was an abrupt decrease in bound apolipoprotein. A similar change in binding occurred using DLPC, although the relatively large changes in bound protein occurred at about 10 and 20 degrees C, temperatures which are above the phase transition temperature of this lipid. Experiments using DOPC were limited to temperatures above the phase transition, and apolipoprotein binding was low. Experiments monitoring the intrinsic fluorescence of the protein, and the fluorescence of bis-1-anilino-8-naphthalene sulfonic acid bound to the protein, revealed a possible conformational change at about 40 degrees C. Measurement of intrinsic fluorescence provided the same result whether or not the protein was associated with lipid. DSC of the apolipoprotein indicated that this change was not associated with a measurable thermogenic process. We found that the interaction with DPPC was reversible at 42 degrees C, and we measured the thermodynamic parameters of the interaction at this temperature. These were: delta G0 = -8.0 kcal/mol apolipoprotein; delta H0 = -88 kcal/mol; delta S0 = -254 cal/Cdeg per mol. We conclude that the interaction between SP 35 and saturated phosphatidylcholines is temperature sensitive, and this probably reflects differences in the ability of gel and liquid-crystalline phospholipids to bind this protein. Both the delta H0 and delta S0 of the interaction are negative, and may reflect an immobilization of phospholipid around the apolipoprotein to form a boundary layer. This hypothesis is consistent with the findings obtained by DSC, in which the enthalpy of the phase transition of DMPC in lipid-apolipoprotein recombinants was found to be about 60% of that expected for a pure and unperturbed multilamellar dispersion.