Regulation of phospholipase C delta 1 by sphingosine

Sphingosine, which is on the pathway of sphingomyelin degradation, activates phospholipase C (PLC) delta 1 moderately. In the liposome assay effect of sphingosine on PLC delta 1 activity depends on KCl concentration. Stimulation of PLC delta 1 by sphingosine increased as the KCl concentration is increased from 0 to 100 mM, and then diminished with the increasing KCl. In the liposome assay sphingosine diminishes inhibition of PLC delta 1 by sphingomyelin; To determine the domain of PLC delta 1 which interacts with sphingosine active proteolytic fragments of PLC delta 1 were generated by trypsin digestion of the native enzyme. Sphingosine affects the activity of PLC delta 1 fragment which lacked the amino-terminal domain (first 60 amino acids) but not the active fragment that has cleaved the domain spanning the X and Y region of PLC delta 1. These observations indicate that for interaction of sphingosine with PLC delta 1 intact domain that span regions of conservation, designated as X and Y is necessary. When the activity of PLC delta 1 was assayed with PIP2 in the erythrocyte membrane as substrate, sphingosine strongly inhibited PLC delta 1. The other homolog of sphingosine 4-hydroxysphinganine (phytosphingosine) inhibited PLC delta 1 to much lesser extent. The activity of PLC delta 1 was inhibited by 68% and 22% in the presence of 20 mu M sphingosine and phytosphingosine, respectively. This inhibition was completely abolished by deoxycholate at a concentration of 1.5 mM. These observations suggest that sphingosine may regulate activity of PLC delta 1 in the cell.