Direct Demonstration of the Activation of UDP‐N‐Acetylgalactosamine: [GM3]N‐Acetylgalactosaminyltransferase by Cyclic AMP

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Abstract
Treatment of NG 108–15 cells in culture with the opiate peptide [D‐Ala2,D‐Leu5]enkephalin produces maximal inhibition of cyclic AMP synthesis in < 15 min. The activity of [GM3]:N‐acetylgalactosaminyltransferase is similarly inhibited, but maximal inhibition is not observed for at least 30 min following the addition of [D‐Ala2,D‐Leu5]enkephalin. Conversely, the phosphodiesterase inhibitor 3‐isobutyl‐1‐methylxanthine rapidly potentiates the intracellular accumulation of cyclic AMP and, in a more gradual fashion, increases [GM3]:N‐acetyl‐galactosaminyltransferase activity. The reductions in the activity of [GM3]:N‐acetylgalactosaminyltransferase that occur following treatment of NG108‐15 cells with indomethacin argues for a direct role of cyclic AMP in the observed changed in [GM3]:N‐acetylgalactosaminyltransferase activity. By adding low concentrations of cyclic AMP (but not cyclic GMP) to microsomes derived from neonatal rat brain, we were able to demonstrate a dose‐dependent phosphorylation of membrane protein and subsequent doubling of [GM3]:N‐acetylgalactosaminyl‐transferase activity.