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 Ca²⁺-ATPase activity yielded an apparent inhibition constant of approx. 0.4 μM. 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 Ca²⁺ concentration with a decreased maximum catalytic rate; (iv) it occurred in the absence of Ca²⁺ ionophoric activity. Likewise, the inhibition mechanism was linked to: (i) rapid enzyme phosphorylation from ATP in the presence of Ca²⁺ 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 Ca²⁺; (iii) decreased phosphoenzyme levels at saturating Ca²⁺ concentrations; (iv) no effect on kinetics of phosphoenzyme decomposition. The palytoxin effect is related with lock of the enzyme in the Ca²⁺-free conformation so that progression of the catalytic cycle is impeded.