The thermal transition in crude myelin proteolipid has a lipid rather than protein origin.

Myelin proteolipid has been isolated from bovine brain and purified using organic solvents according to conventional procedures. The protein content of the purified sample, or crude proteolipid, contains a minimum of 75% w/w of proteolipid, with DM-20, a proteolipid molecule with an internal deletion of 35 out of 276 amino acid residues, as the only other component. Biochemical analysis has shown the differences in lipid composition between brain white matter, myelin and crude proteolipid preparations. The latter contained practically no cholesterol, while the other two samples had about 22-23% w/w. High-sensitivity differential scanning calorimetry experiments with both crude proteolipid and its extracted pool of lipids have shown similar reversible thermal transitions at 52 degrees C and 48 degrees C. The effect of increasing amounts of cholesterol on the two calorimetric transitions led in both cases to a continuous decrease in the melting temperature and in the transition enthalpy. Parallel Fourier-transform infrared spectroscopy studies of crude proteolipid have detected a reversible, co-operative lipid transition centred at 49 degrees C, with no detectable change in the amide region between 20 degrees C and 60 degrees C. Once more an increase in cholesterol content led to a decrease in the sharpness of this transition. It is concluded that the thermal transition detected in crude proteolipid, which has in the past been attributed to proteolipid thermal denaturation (Mateo et al. 1986), actually corresponds to a thermotropic phase transition of the lipids included in the crude proteolipid sample.