Biosynthesis of sphingolipids: dihydroceramide and not sphinganine is desaturated by cultured cells.

Radioactively labeled N-[1-14C]-octanoyl-sphinganine and D-erythro-[3-3H]-sphinganine were administered in parallel experiments to neuroblastoma cells B 104. A time dependent formation of ceramide with a double bond in its sphingoid backbone was observed in both cases. In the presence of fumonisin B1 (25 microM), a strong inhibitor of sphinganine N-acyltransferase, desaturated ceramide was formed only when cells were fed with N-[1-14C]-octanoyl-sphinganine but not with [3-3H]-sphinganine. Thus, the introduction of the double bond occurs only at the level of dihydroceramide, after N-acylation of sphinganine. It is now obvious that sphingosine is not a biosynthetic intermediate but exclusively a catabolic product of cellular sphingolipids.