Inhibition of protein kinase C by sphingosine correlates with the presence of positive charge

The role of the 2-amino group of sphingosine on the in vitro inhibition of protein kinase C was investigated by comparing protein kinase C activity in the presence and absence of sphingosine at various pH's. Inhibition by sphingosine was found to be pH dependent. Above pH 7.75, sphingosine has little or no inhibitory effect. In fact, at pH 8.5, sphingosine slightly enhances enzyme activity above that which occurs when the enzyme is stimulated by diacylglycerol and phosphatidylserine. After correcting for electrostatic repulsion, we find that the intrinsic pK for sphingosine in Triton micelles is 8.5. Inhibition of protein kinase C by sphingosine at physiological pH's therefore correlates with the presence of a positive charge.