Mechanism of protein kinase C inhibition by sphingosine

The in vitro mechanism by which sphingosine inhibits protein kinase C (PKC) was investigated by comparing enzyme activity and the physical associations of reaction components. Light scattering intensity measurements showed that sphingosine prevented the association of the substrate, histone, with micelles of Triton plus phosphatidylserine (PS). Addition of phosphatidylinositol (PI) or phosphatidylglycerol (PG) restored histone interaction. In direct correlation, both PI and PG were able to reverse inibition of PKC activity by sphingosine. In Triton mixed micelles, neither PI nor PG alone would support PKC activity or substrate-lipid binding. Inhibition of PKC by positively charged sphingosine appeared to be related to simple charge neutralization of the lipid, thereby preventing interaction with PKC and/or its protein substrate.