BIOSYNTHESIS OF GANGLIOSIDES CONTAINING C18 1 AND C20/1 [3-C-14]SPHINGOSINE AFTER ADMINISTRATING [1-C-14]PALMITIC ACID AND [1-C-14]STEARIC ACID TO RAT CEREBELLAR GRANULE CELLS IN CULTURE/

The biosynthesis of ganglioside molecular species containing sphingosi ne of different structure was investigated by administrating rat cereb ellar granule cells in culture with [1-C-14]palmitic and [1-C-14]stear ic acids which are the precursors for sphingosine biosynthesis. The in corporation of radioactivity into the sphingosine of the ganglioside s pecies containing C20:1 sphingosine after administrating [1-C-14]stear ic acid was low in comparison with the incorporation of radioactivity into the sphingosine of ganglioside species containing C18:1 sphingosi ne after administration of [1-C-14]palmitic acid, but the ratio betwee n the radioactivity incorporated in the C20:1 and the C18:1 sphingosin e of C20 and C18 ganglioside species progressively increased when the cell culture was prolonged. Ceramide-containing radioactive sphingosin e was found after palmitic or stearic acid administration. Ceramide-co ntaining C20:1 sphingosine found after adding stearic acid was about 5 % of that synthesized starting from palmitic acid and containing C18:1 sphingosine. Free radioactive C18:1 and C20:1 sphingosine were found after adding radioactive palmitic or stearic acid. This is representat ive of a catabolic process occurring after biosynthesis of the complex sphingolipid starting from the radioactive precursor. In fact it has been proved that only saturated sphingosines are used for the synthesi s of complex sphingolipids, the introduction of the double bond at pos ition four of the sphingoid chain occurring at the level of ceramide [ Rother, J., van Echten, G., Schwarzmann, G. and Sandhoff, K. (1992) Bi ochem. Biophys. Res. Commun. 189, 14-20]. Saturated sphingosines were not present. The lack of free C20:0 sphingosine confirms the hypothesi s that the C20:0 sphingosine synthesis and the process (C20:0 sphingos ine-->C20:0 ceramide-->C20:1 ceramide) occur in the correct quantity f or the synthesis of C20:1 gangliosides. Moreover, we found only traces of free C20:1 sphingosine, at days 8 and 15 of cell culture when the biosynthesis of complex C20:1 gangliosides and the related catabolic p rocesses occur to a higher extent, thus excluding the idea that a larg e amount of C20:0 sphingosine can be acylated to C20:0 ceramide and de hydrogenated to C20:1 ceramide which, being not used for ganglioside b iosynthesis, is immediately catabolised to C20:1 sphingosine.