THE THERMAL TRANSITION IN CRUDE MYELIN PROTEOLIPID HAS A LIPID RATHERTHAN PROTEIN ORIGIN

Myelin proteolipid has been isolated from bovine brain and purified us ing organic solvents according to conventional procedures. The protein content of the purified sample, or crude proteolipid, contains a mini mum 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 onl y other component. Biochemical analysis has shown the differences in l ipid composition between brain white matter, myelin and crude proteoli pid preparations. The latter contained practically no cholesterol, whi le the other two samples had about 22-23% w/w. High-sensitivity differ ential scanning calorimetry experiments with both crude proteolipid an d its extracted pool of lipids have shown similar reversible thermal t ransitions 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 proteol ipid thermal denaturation (Mateo et al. 1986), actually corresponds to a thermotropic phase transition of the lipids included in the crude p roteolipid sample.