Inhibition of the intracellular Ca2+ transporter SERCA (Sarco-Endoplasmic Reticulum Ca2+-ATPase) by the natural polyphenol epigallocatechin-3-gallate

The use of a microsomal preparation from skeletal muscle revealed that both Ca2+ transport and Ca2+-dependent ATP hydrolysis linked to Sarco-Endoplasmic Reticulum Ca2+-ATPase are inhibited by epigallocatechin-3-gallate (EGCG). A half-maximal effect was achieved at approx. 12 mu M. The presence of the galloyl group was essential for the inhibitory effect of the catechin. The relative inhibition of the Ca2+-ATPase activity decreased when the Ca2+ concentration was raised but not when the ATP concentration was elevated. Data on the catalytic cycle indicated inhibition of maximal Ca2+ binding and a decrease in Ca2+ binding affinity when measured in the absence of ATP. Moreover, the addition of ATP to samples in the presence of EGCG and Ca2+ led to an early increase in phosphoenzyme followed by a time-dependent decay that was faster when the drug concentration was raised. However, phosphorylation following the addition of ATP plus Ca2+ led to a slow rate of phosphoenzyme accumulation that was also dependent on EGCG concentration. The results are consistent with retention of the transporter conformation in the Ca2+-free state, thus impeding Ca2+ binding and therefore the subsequent steps when ATP is added to trigger the Ca2+ transport process. Furthermore, phosphorylation by inorganic phosphate in the absence of Ca2+ was partially inhibited by EGCG, suggesting alteration of the native Ca2+-free conformation at the catalytic site.