Characterization of the palytoxin effect on Ca2+-ATPase from sarcoplasmic reticulum (SERCA).
The effect of palytoxin was studied in a microsomal fraction enriched in longitudinal tubules of the sarcoplasmic reticulum membrane. Half-maximal effect of palytoxin on Ca(2+)-ATPase activity yielded an apparent inhibition constant of approx. 0.4 microM. The inhibition process exhibited the following characteristics: (i) the degree of inhibition was dependent on membrane protein concentration; (ii) no protection was observed when the ATP concentration was raised; (iii) dependence on Ca(2+) concentration with a decreased maximum catalytic rate; (iv) it occurred in the absence of Ca(2+) ionophoric activity. Likewise, the inhibition mechanism was linked to: (i) rapid enzyme phosphorylation from ATP in the presence of Ca(2+) but lower steady-state levels of phosphoenzyme; (ii) more drastic effect on phosphoenzyme levels when the toxin was added to the enzyme in the absence of Ca(2+); (iii) decreased phosphoenzyme levels at saturating Ca(2+) concentrations; (iv) no effect on kinetics of phosphoenzyme decomposition. The palytoxin effect is related with lock of the enzyme in the Ca(2+)-free conformation so that progression of the catalytic cycle is impeded.
Coca
Ramón
R
Departamento de Bioquímica y Biología Molecular A, Universidad de Murcia, Campus de Espinardo, 30071 Espinardo, Murcia, Spain.
Soler
Fernando
F
Fernández-Belda
Francisco
F
D000178:Acrylamides / Q000494:pharmacology*; D000255:Adenosine Triphosphate / Q000737:chemistry; D000818:Animals; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002384:Catalysis; D002462:Cell Membrane / Q000378:metabolism; D003064:Cnidarian Venoms; D005260:Female; D007476:Ionophores / Q000494:pharmacology; D007700:Kinetics; D008954:Models, Biological; D010766:Phosphorylation; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000378:metabolism; D000095702:Polyether Toxins
D000178:Acrylamides; D000255:Adenosine Triphosphate; D002118:Calcium; D000252:Calcium-Transporting ATPases; D003064:Cnidarian Venoms; D007476:Ionophores; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; C010272:palytoxin; D000095702:Polyether Toxins