Activation of casein kinase II by sphingosine

Sphingosine activates casein kinase II in the presence of endogenous substrates as well as a synthetic peptide substrate. The activation response occurred between 12 and 25 μg/ml sphingosine and exhibited positive cooperativity with a Hill coefficient of 3.0. Sphingosine not only increased the V_max of casein kinase II but decreased the K_m(app) for the peptide substrate from 0.5 to 0.08 mM. In contrast, the K_m(app) for MgCl₂ was increased from 0.12 to 0.7 mM. Consequently, sphingosine altered significantly several parameters which determine casein kinase II activity. The effect of sphingosine was relatively specific, inasmuch as related lipids were less potent activators or largely ineffective in stimulating casein kinase II. On the other hand, the effect of sphingosine itself could be potentiated or inhibited by other lipids. Ceramide and sphingosylphosphorylcholine augmented the sphingosine effect. Phospholipids alone did not alter the activity of casein kinase II significantly, but abolished enzyme activation by sphingosine with different potencies (phosphatidylserine > phosphatidylethanolamine > phosphatidylinositol > phosphatidylcholine). Moreover, the sphingosine effect could be abrogated by KCl and NaCl, which alone are known to induce enzyme activation and dissociation of aggregated casein kinase II protein; LiCl and NH₄Cl also inhibited the sphingosine effect. Polyamines, known activators of casein kinase II, partially mimicked the effect of sphingosine on endogenous polypeptide phosphorylation but failed to do so with the peptide substrate. These observations demonstrate that sphingosine is a potent activator of casein kinase II. The potential pharmacological and physiological modulation of casein kinase II by sphingoid bases is discussed.