Thermal behavior of fractionated and unfractionated bovine brain cerebrosides

Bovine brain cerebrosides have been fractionated into 2-hydroxy fatty acid containing cerebrosides (HFA-CER) and nonhydroxy fatty acid containing cerebrosides (NFA-CER). The thermal behavior of NFA-CER, HFA-CER, and unfractionated cerebroside model membranes has been studied by differential scanning calorimetry. When NFA-CER is cooled at rates greater than or equal to 2.5 degrees C/min, subsequent heating runs exhibit metastable behavior: a low enthalpy exotherm is observed at approximately 50 degrees C (delta H = -(1-3) cal/g), followed by a high enthalpy endotherm at 72 degrees C (delta H = 16-17 cal/g). Systematic variation of cooling/heating protocols indicates that NFA-CER possesses two low-temperature states, one metastable and the other stable. Cooling from the liquid-crystalline state results in formation of the metastable low-temperature polymorph I, which must transform into the stable low-temperature polymorph II before the liquid-crystalline state can be reached again. By analogy with recent X-ray studies of synthetic N-palmitoylgalactosylsphingosine [Ruocco, M. J., Atkinson, D., Small, D. M., Skarjune, R. P., Oldfield, E., & Shipley, G. G. (1981) Biochemistry 20, 5957], it is proposed that metastable polymorph I is dehydrated relative to stable polymorph II. HFA-CER displays no metastability and exhibits a reversible thermal transition at approximately 68 degrees C (delta H = 7.3 cal/g). The thermal behavior of unfractionated cerebrosides is similar to that of HFA-CER, exhibiting a single reversible transition at approximately 67 degrees C (delta H = 6.9 cal/g). These results suggest that a function of hydroxy fatty acids in brain cerebrosides may be to prevent metastable dehydration in the cerebroside-rich myelin membrane.