Possible involvement of sphingomyelin in the regulation of the plasma sphingosine 1-phosphate level in human subjects.

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid mediator. Although the plasma S1P concentration is reportedly determined by cellular components, including erythrocytes, platelets, and vascular endothelial cells, the possible involvement of other factors, such as serum sphingomyelin (SM) and autotaxin (ATX), remains to be elucidated. We measured S1P using high-performance liquid chromatography (HPLC), SM and lysophosphatidic acid (LPA) using enzymatic assays, ATX antigen using a two-site enzyme immunoassay, and ATX activity using a lysophospholipase D activity assay. To fractionate the lipoproteins, plasma samples were separated using fast protein liquid chromatography (FPLC) utilizing a Superose 6 column. The plasma S1P level was positively correlated with the levels of SM and lysophosphatidylcholine, but not with the level of phosphatidylcholine. Although SM was present in the very low-density lipoprotein (VLDL) fraction, neither the plasma S1P level nor the SM level was affected by feeding. The plasma S1P level was negatively correlated with the ATX activity. Although the incubation of 100 μmol/L of sphingosylphosphorylcholine (SPC) with the serum resulted in a significant increase in the S1P level because of the presence of ATX, the physiological concentration of SPC did not mimic this effect. The plasma S1P level was affected by the serum SM level, while the possibility of ATX involvement in the increase in the plasma S1P level was considered to be remote at least in healthy human subjects.