EFFECT OF MEMBRANE-LIPIDS ON THE LACTOSYLCERAMIDE MOLECULAR-SPECIES SPECIFICITY OF CMP-N-ACETYLNEURAMINATE, LACTOSYLCERAMIDE SIALYLTRANSFERASE

It has previously been shown that when the molecular species specifici ty of rat liver Golgi CMP-N-acetylneuraminate:lactosylceramide alpha2, 3-sialyltransferase was determined, using as the substrate lactosylcer amide (LacCer) incorporated into liposomes prepared with rat liver Gol gi lipids, the enzyme showed a pronounced variation in activity toward s the various molecular species of LacCer (J. Lipid Res. 1989. 30: 178 9-1797). In this paper, the LacCer molecular species specificity of si alyltransferase from neuroblastoma NB2a cells was examined using five naturally occurring and three synthetic molecular species of LacCer. T he enzyme activity was determined by following the formation of [C-14] GM3 from CMP-[C-14]neuraminic acid and individual molecular species of LacCer incorporated into liposomes. Nonspecific lipid transfer protei n was included in the enzyme assay to facilitate the transfer of LacCe r and other lipids between the liposomes and the membrane where sialyl transferase is located. In these enzyme assays the liposomes contained approximately 10 times more lipid phosphorus than either the microsom al fraction of NB2a cells or the Golgi fraction of rat liver. Thus, in the presence of nonspecific lipid transfer protein, the lipid composi tion of the membrane where sialyltransferase is located was modified t o resemble the lipid composition of the liposomes. When the molecular species specificity of NB2a cell sialyltransferase was determined with LacCer incorporated into liposomes prepared with NB2a cell lipids, th e enzyme showed no specificity towards the various molecular species o f LacCer. However, when the molecular species specificity of NB2a cell sialyltransferase was determined with LacCer incorporated into liposo mes prepared with rat liver Golgi lipids, the enzyme showed a variatio n in activity towards the various LacCer molecular species similar to that observed with the liver Golgi enzyme using liposomes prepared wit h liver Golgi lipids. Likewise, when the molecular species specificity of rat liver Golgi sialyltransferase was determined with LacCer incor porated into liposomes prepared with NB2a cell lipids, the liver enzym e then showed no specificity towards the various molecular species of LacCer. These results indicate that the lipid environment of the membr ane can alter the molecular species specificity of sialyltransferase t owards its lipid substrate, LacCer.