Biosynthesis of Sphingolipid Bases

In the absence of reduced triphosphopyridine nucleotide, two isotopically labeled ketones were found to accumulate when l-serine and ammonium palmitate-14C were incubated with ATP, coenzyme A, and a cell-free particulate preparation from the yeast Hansenula ciferri. These compounds, in the form of their N-acetyl-, trimethylsilyl-, and 2,4-dinitrophenylhydrazone derivatives, have been identified as 3-ketodihydrosphingosine (1-hydroxy-3-oxo-2-aminooctadecane) and 3-ketosphingosine (1-hydroxy-3-oxo-2-aminooctadec-4-ene) by thin layer chromatography and gas-liquid chromatography, and by repeated crystallizations of N-acetyl-3-ketodihydrosphingosine to constant specific radioactivity. In the presence of TPNH, these compounds are enzymatically reduced to dihydrosphingosine and sphingosine. These findings indicate that 3-ketodihydrosphingosine and 3-ketosphingosine are intermediates in the biosynthesis of dihydrosphingosine and sphingosine, respectively, and that in yeast the biosynthetic pathways leading to saturated and unsaturated sphingolipid bases diverge at some point before reduction of the ketonic intermediates with TPNH.