Modulation of phosphorylase kinase activity by sphingolipids

Psychosine (galactosyl sphingosine) potently inhibits the activity of both nonactivated and activated by covalent modification (autophosphorylation and limited proteolysis) rabbit skeletal muscle phosphorylase kinase. Half-maximal inhibition was observed at 44 microM or 66 microM when the kinase activity was assayed at pH 6.8 or 8.2 respectively. Sphingosine was also inhibitory, but only at pH 6.8 (half-maximal inhibition was observed at 130 microM). In this respect, sphingomyelin, cerebroside and cerebroside sulfate were ineffective. On the other hand, a number of gangliosides stimulated the activity of nonactivated phosphorylase kinase at neutral pH. Among the individual gangliosides tested the activation potency was GD1a greater than GT1b greater than GM1, while GM3 was without effect. Most important, GD1a dramatically increases the activity of the kinase at low Ca2+ concentrations. Both psychosine and GD1a increased the rate of kinase autophosphorylation on alpha- subunit only, but although ganglioside-induced stimulation of autophosphorylation was accompanied with an enhancement of the rate of autoactivation at pH 6.8, psychosine completely blocked autoactivation.