DOMOIC ACID INHIBITS ADENYLATE-CYCLASE ACTIVITY IN RAT-BRAIN MEMBRANES

Citation
Ms. Nijjar et B. Grimmelt, DOMOIC ACID INHIBITS ADENYLATE-CYCLASE ACTIVITY IN RAT-BRAIN MEMBRANES, Molecular and cellular biochemistry, 136(2), 1994, pp. 105-111
Citations number
30
Categorie Soggetti
Biology
ISSN journal
03008177
Volume
136
Issue
2
Year of publication
1994
Pages
105 - 111
Database
ISI
SICI code
0300-8177(1994)136:2<105:DAIAAI>2.0.ZU;2-P
Abstract
Adenylate cyclase activity measured by the formation of cyclic AMP in rat brain membranes was inhibited by a shellfish toxin, domoic acid (D OM). The inhibition of enzyme was dependent on DOM concentration, but about 50% of enzyme activity was resistant to DOM-induced inhibition. Rat brain supernatant resulting from 105,000 x g centrifugation for 60 min, stimulated adenylate cyclase activity in membranes. Domoic acid abolished the supernatant-stimulated adenylate cyclase activity. The b rain supernatant contains factors which modulate adenylate cyclase act ivity in membranes. The stimulatory factors include calcium, calmoduli n, and GTP, In view of these findings, we examined the role of calcium and calmodulin in DOM-induced inhibition of adenylate cyclase in brai n membranes. Calcium stimulated adenylate cyclase activity in membrane s, and further addition of calmodulin potentiated calcium-stimulated e nzyme activity in a concentration dependent manner. Calmodulin also st imulated adenylate cyclase activity, but further addition of calcium d id not potentiate calmodulin-stimulated enzyme activity. These results show that the rat brain membranes contain endogenous calcium and calm odulin which stimulate adenylate cyclase activity. However, calmodulin appears to be present in membranes in sub-optimal concentration for a denylate cyclase activation, whereas calcium is present at saturating concentration. Adenylate cyclase activity diminished as DOM concentrat ion was increased, reaching a nadir at about 1 mM. Addition of calcium restored DOM-inhibited adenylate cyclase activity to the control leve l. Similarly, EGTA also inhibited adenylate cyclase activity in brain membranes in a concentration dependent manner, and addition of calcium restored EGTA-inhibited enzyme activity to above control level. The f act that EGTA is a specific chelator of calcium, and that DOM mimicked adenylate cyclase inhibition by EGTA, indicate that calcium mediates DOM-induced inhibition of adenylate cyclase activity in brain membrane s. While DOM completely abolished the supernatant-, and Gpp(NH)p-stimu lated adenylate cyclase activity, it partly blocked calmodulin-, and f orskolin-stimulated adenylate cyclase activity in brain membranes. The se results indicate that DOM may interact with guanine nucleotide-bind ing (G) protein and/or the catalytic subunit of adenylate cyclase to p roduce inhibition of enzyme in rat brain membranes.