Attenuation of Enkephalin Activity in Neuroblastoma × Glioma NG108—15 Hybrid Cells by Phospholipases

Abstract: The role of membrane phospholipids in enkephalin receptor‐mediated inhibition of adenylate cyclase (EC 4.6.1.1) activity in neuroblastoma × glioma NG108‐15 hybrids was studied by selective hydrolysis of lipids with phospholipases. When NG108‐15 cells were treated with phospholipase C from Clostridium welchii at 37°C, an enzyme concentration‐dependent decrease in adenylate cyclase activity was observed. The basal and prostaglandin E₁ (PGE₁)‐stimulated adenylate cyclase activities were more sensitive to phospholipase C (EC 3.1.4.3) treatment than were the NaF‐5′‐guanylyl‐imidodiphosphate (Gpp(NH)p)‐sensitive adenylate cyclase activities. Further, Leu⁵‐enkephalin inhibition of basal or PGE₁‐stimulated adenylate cyclase activity was attenuated by phospholipase C treatment, characterized by a decrease of enkephalin potency and of maximal inhibitory level. [³H]d‐Ala²‐Met⁵‐enkephalinamide binding revealed a decrease in receptor affinity with no measurable reduction in number of binding sites after phospholipase C treatment. Although opiate receptor was still under the regulation of guanine nucleotide after phospholipase C treatment, adenylate cyclase activity was more sensitive to the stimulation of Gpp(NH)p. Thus, the reduction of opiate agonist affinity was not due to the uncoupling of opiate receptor from N‐component. Further, treatment of NG108‐15 hybrid cell membrane with phospholipase C at 24°C produced analogous attenuation of enkephalin potency and efficacy without alteration in receptor binding. The reduction in enkephalin potency could be reversed by treating NG108‐15 membrane with phosphatidylcholine, but not with phosphatidylserine, phosphatidylinositol, or cerebroside sulfate. The enkephalin activity in NG108‐15 cells was not altered by treating the cells with phospholipase A₂ or phospholipase C from Bacillus cereus. Hence, apparently, there was a specific lipid dependency in enkephalin inhibition of adenylate cyclase activity.