Metabolism of free sphingoid bases in murine tissues and in cultured human fibroblasts.

Metabolism of free sphingoid bases present in normal tissues [Kobayashi, T., Mitsuo, K. & Goto, I. (1988) Eur. J. Biochem. 172, 747-752] was examined in mouse tissues and in human cultured fibroblasts. Subcellular fractionation studies of the mouse liver revealed most of free sphingoid bases to be in the membrane fractions. Fibroblasts from patients with Farber's disease contained concentrations of free sphingoid bases similar to those seen in the normal fibroblasts. When L-cycloserine, a potent inhibitor of the first reaction of sphingoid base synthesis, was added to the culture medium, the concentration of free sphingoid bases in fibroblasts decreased, dose-dependently. Thus, most of the free sphingoid bases in the tissue probably do not derive from the degradation of sphingolipids but are newly synthesized. Free sphingoid bases in microsomes from the brainstem and spinal cord were acylated or glycosylated when incubated with acyl-CoA or UDP-glycoside. However, the reaction for the synthesis of ceramide was much greater and more rapid than that of glycosylsphingosine synthesis. In liver microsomes, ceramide synthesis from endogenous free sphingoid bases was observed but synthesis of glycosylsphingosine was not evident. Therefore, the main pathway of metabolism of free sphingoid bases is presumably acylation (to ceramide) but not glycosylation (to lysoglycosphingolipid).