Glycolipids Isolated from Aplysia kurodaiCan Activate Cyclic Adenosine 3′, 5′‐Monophosphate‐Dependent Protein Kinase from Rat Brain

Abstract: Cyclic AMP (cAMP)‐dependent protein kinase (cAMP‐kinase) partially purified from the membrane fractions of rat brains was stimulated by novel phosphonogly‐cosphingolipids (glycolipids) derived from the skin and nerve fibers of Aplysia kurodai. Among various glycolipids tested, a major glycolipid from the skin, 3‐O‐MeGalβ 1→3GalNAcα 1→3 [6′‐O‐(2‐aminoethylphosphonyl) Galα1→2] (2‐aminoethylphosphonyl→6) Glcβ 1→4GICβ1→1ceramide (SGL‐II), was most potent, giving half‐maximal activation at 32.2 μM. Activation of cAMP‐kinase was maximal with 250 μM SGL‐II using kemptide as substrate. The effect of SGL‐II was additive on kinase activity at submaximal concentrations of cAMP. The kinase activity activated with SGL‐II was inhibited by the addition of protein kinase inhibitor peptide, a specific peptide inhibitor for cAMP‐kinase. Its inhibitory pattern was similar to that for the catalytic subunit. Of the various substrates tested, the glycolipid‐stimulated cAMP‐kinase could phosphorylate microtubule‐associated protein 2, synapsin I, and myelin basic protein but not histone H1 and casein. The regulatory subunit strongly inhibited the activity of purified catalytic subunit of cAMP‐kinase. This inhibition was reversed by addition of SGL‐II, as observed for cAMP. SGL‐II was capable of partially dissociating cAMP‐kinase, which was observed by gel filtration column chromatography. However, the binding activity of cAMP to the holoenzyme was not inhibited with SGL‐II. These results demonstrate that the glycolipids can directly activate cAMP‐kinase in a manner similar, but not identical, to that of cAMP.