LACTOSYLCERAMIDE IS SYNTHESIZED IN THE LUMEN OF THE GOLGI-APPARATUS

Recently, synthesis of lactosylceramide has been described to occur on the cytosolic face of the Golgi [(1991) J. Biol. Chem. 266, 20907-209 12]. The reactions following in the biosynthesis of higher glycosphing olipids are known to take place in the lumen of the Golgi. For our und erstanding of the functional organization of the multiglycosyltransfer ase system of glycosphingolipid synthesis in the Golgi, the knowledge of the topology of individual reactions is a prerequisite. We have dev eloped a simple and quick assay system for sphingolipid biosynthesis a nd have obtained evidence that lactosylceramide is synthesized in the lumen of the Golgi. Because lactosylceramide is generated by galactosy lation of glucosylceramide which, in turn, is synthesized from ceramid e and UDP-Glc on the cytosolic surface of the Golgi apparatus, further efforts will be directed to the characterization of a glucosylceramid e-translocator in the Golgi membranes rather than a lactosylceramide-t ranslocator.