Ganglioside Modulation of Cyclic AMP‐Dependent Protein Kinase and Cyclic Nucleotide Phosphodiesterase In Vitro

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Abstract
Purified cyclic AMP‐dependent protein kinase (cAK) catalytic subunit phosphorylated 180‐, 49‐, 31‐, 19‐, and 14‐kilodalton (kDa) proteins of rabbit sciatic nerve membranes. The ability of cAK to phosphorylate these membrane substrate proteins was inhibited by gangliosides GM1, GDIa, and GTlb with half‐maximal inhibitory concentration (I50) = 7–25 μM. Neutral glycolipids and lyso‐phosphatidylcholine were much less effective. Cyclic AMP (cAMP) kinase phosphorylation of histone IIA was inhibited by GM1, GDla, and GTlb (I50 = 115 μM, 75 μM, and 75 μM, respectively). Inhibition by GM1 was competitive with respect to histone (K‐, = 108 μM). Autophosphorylation of cAMP kinase was inhibited by GM1 (I50 = 15 μM). GTlb, GDla, and GM1 half‐maximally stimulated calmodulin‐de‐pendent cyclic nucleotide phosphodiesterase at 0.1 μM, 0.2 μM, and 0.3 μM, respectively. Although GTlb stimulated phosphodiesterase by increasing Vmax and decreasing Km (similar to calmodulin), GDla and GM1 produced only an increase in Vmax. These results suggest that ganglioside can modulate the activity of cAMP kinase by both direct inhibition of the enzyme and indirect reduction of cAMP levels through activation of phosphodiesterase. Through these mechanisms, gangliosides may alter cAMP‐dependent protein phosphorylation and cell function within the nervous system.