FIGURE 6.

A, C₁₈- and C₁₆-SO levels in human plasma. Plasma of 20 healthy donors were acid/base-treated and analyzed by LC-MS. The analysis revealed that the majority of plasma sphingolipids are based on a SO backbone, whereas about 4% of the plasma sphingolipids contain a SA backbone. Up to 15% of the plasma sphingosine and sphinganine fraction was based on a C₁₆ backbone. B, C₁₈- and C₁₆-SO are components of plasma LDL and HDL lipoprotein fractions. Human plasma was fractionated by a four-step density gradient ultracentrifugation. The individual fractions were assayed for cholesterol (Chol, dotted line), triglycerides (TG, dashed line), C₁₈-SO (rhombus), and C₁₆-SO (square). The cholesterol and TG profile indicated the peak for LDL in fraction 4 and for HDL in fractions 7 and 8. In parallel the highest C₁₈-SO and C₁₆-SO concentrations were found in these fractions. The elevated TG concentrations in fraction 1 indicated the presence of VLDL, which did not contain C₁₈-SO or C₁₆-SO. The sum of all fractions was defined as 100%, and values are given in percent of total.