Similar effect of proteolipid apoproteins from human myelin (lipophilin) and bovine white matter on the lipid phase transition.

The proteolipid apoprotein from bovine white matter has been reported to increase the phase transition temperature of dimyristoyl phosphatidylcholine, in contrast to a proteolipid apoprotein fraction from human myelin, called lipophilin, which decreases the enthalpy without altering the phase transition temperature. Since these results lead to different conclusions concerning the structure and amount of boundary lipid surrounding these proteins, the effects of the two proteins on the phase transition of dimyristoyl phosphatidylcholine were compared. Neither protein has any effect on the phase transition temperature, regardless of the method of delipidation of the protein, the amount of residual lipid, the method of incorporation into vesicles, or heating rates used for differential scanning calorimetry. However, a higher melting component was observed when decomposition of the lipid to lysophosphatidylcholine had occurred. Addition of as little as 6% of the decomposition products of dimyristoyl phosphatidylcholine, lysodimyristoyl phosphatidylcholine and myristic acid, is enough to produce a higher-temperature peak. The intensity of this peak increases with increasing protein concentration similar to the reported result on the bovine white matter proteolipid. The question as to whether the protein-induced decrease in enthalpy is due to boundary lipid or entrapment of lipid in protein aggregates was also addressed by studying the appearance of the intramembranous protein particles by freeze-fracture electron microscopy at temperatures above and below the phase transition and between the premelt and main transitions. The protein is randomly dispersed above the phase transition. At low concentrations, below the phase transition, it clusters, forming particle-free and particle-rich areas, but does not aggregate. At higher concentrations it is randomly dispersed below the premelt and main transition but is clustered between the premelt and main transition. Since the protein is more randomly dispersed above the transition than below, the reduction in enthalpy of the freezing transition was compared to that of the melting transition and was found to be identical, suggesting that the withdrawal of lipid from the phase transition is probably not due to lipid entrapment but due to the formation of a boundary lipid interface between the protein and the bulk lipid.