On the accessibility and localisation of cerebrosides in central nervous system myelin

Cerebrosides are concentrated in the myelin sheath where they account for about 20% of the total lipid of the membrane. The present paper is concerned with the role and localisation of these glycolipids in the myelin lamellae. Isolated central nerve myelin preparations have been treated with two probes to investigate cerebroside accessibility in the membrane. The action of galactose oxidase on the galactose headgroups of cerebrosides is followed and quantitated by recovery of the modified glycolipid and resolution of either the 6-aldehydo sugar or galactose remaining by gas-liquid chromatography. With isolated myelin preparations only some 40--50% of the cerebroside galactose is attacked by galactose oxidase at 20 degrees C. With periodate at 20 degrees C over 90% of the galactose headgroups are oxidised in 3 h while the figure is 50--55% over the same time period at 4 degrees C rising to 85% after 22 h. With multilamellar liposomes of mixed myelin lipids only some 20--25% of the available cerebroside is oxidised at 4 degrees C, the reaction being complete in 2 h. The results are discussed in relation to the dispositon of cerebroside in the myelin lamellae. A major location on the external face of the membrane system (intraperiod dense line) is favoured. A role for cerebroside in myelin in terms of increasing the stability and resistance of the lipid phase to ion movement is suggested.