Sphingosine is formed through ceramidase catalyzed hydrolysis of ceramide. It is a potent inhibitor of protein kinase C (IC50 = 1-3 μM). Sphingosine is phosphorylated by sphingosine kinase to the bioactive lipid sphingosine 1-phosphate. Sphingosine-Cyanine5 is labeled with the red-shifted Cy5 dye for measurement and imaging with 646 nm excitation.