Predominance of the acylation route in the metabolic processing of exogenous sphingosine in neural and extraneural cells in culture

The metabolic fate of exogenous [H-3]sphingosine was investigated in five types of cultured cells: primary cultures of neurons and astrocytes, murine and human neuroblastoma cells and human skin fibroblasts. After administration of 40 nM [3-H-3]sphingosine into a cell-conditioned medium containing fetal calf serum, all cell types rapidly and efficiently incorporated the long-chain base in a time-dependent fashion. In all cases, after a 120 min pulse, the amount of radioactivity taken up was in the range of the endogenous sphingosine content. However, unchanged [H-3]sphingosine represented only a very minor portion of the label incorporated into cells throughout the pulse period (10-120 min), indicating rapid and efficient sphingosine metabolism in these cells. Most of the [H-3]sphingosine taken up was metabolically processed, either by degradation (assessed as (H2O)-H-3 release into the culture medium) or by N-acylation (mainly to radioactive ceramide, sphingomyelin, neutral glycolipids and gangliosides). [H-3]Sphingosine 1-phosphate accounted for less than 2 % of the total radioactivity incorporated in all cases. Throughout the pulse period and in all cell types, H-3-labelled organic metabolites largely prevailed over (H2O)-H-3, indicating that N-acylation is the major metabolic fate of sphingosine in these cells under apparently physiological conditions. These results are consistent with the notion that sphingosine has a rapid turnover in the cells studied, and indicate that regulation of the basal level of this bioactive molecule occurs mainly through N-acylation.