Investigation of plant-derived phenolic compounds as plasma membrane Ca2+-ATPase inhibitors with potential cardiovascular activity

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Various flavonoids and alkylphenols have been reported to inhibit the Ca2+-ATPase of plasma membrane (PM) and sarcoplasmic reticulum Ca2+-ATPase. This article describes the development of phenolic inhibitors of PM Ca2+-ATPase as potential cardiovascular agents. The new inhibitors were obtained from the screening of alkylphenols, alkylresorcinols, and phenolic constituents of medicinal plants, followed by synthesis of analogues and analysis of their structure–activity relationships. Alkylphenols were found to inhibit the PM Ca2+-ATPase, with 2-nonylphenol being the most potent compound (IC50 = 30 μM) among a series of synthetic phenols. Several bis-resorcinols were isolated from the Australian plants Grevillea robusta and Grevillea striata, including two novel compounds which were identified as 1-(1,3-dihydroxy-4-methylphenyl)-14-(1,3-dihydroxyphenyl)tetradec-8-Z-ene (grebustol-A) and norstriatol-B. Among these bis-resorcinols, striatol was found to be the most potent inhibitor of PM Ca2+-ATPase, with an IC50 value of 16 μM. Analogues of bis-phenols and bis-resorcinols were synthesised and investigated for Ca2+-ATPase inhibitory activity. It was found that 2,2′-(decane-1,10-diyl)bisphenol (decanediyl-bisphenol) was the most potent inhibitor (IC50 = 10 μM). The synthetic bis-phenols and alkylphenols were shown to inhibit the PM Ca2+-ATPase in a concentration- and structure-dependent fashion. The inhibitory activity of decanediyl-bisphenol was also demonstrated in reconstituted purified Ca2+-ATPase from human erythrocyte plasma membrane and in intact red blood cells and smooth muscle cells. The mechanism of action of these inhibitors, however, is not fully understood but presumably, due to their high lipophilic nature, they would interact with the hydrophobic interior or at the lipid–protein interface, thereby perturbing the Ca2+-ATPase enzyme function. The inhibition was independent of calmodulin stimulation. The inhibition of Ca2+-ATPase was selective relative to the Na+,K+-ATPase and Mg2+-ATPase in the same membrane preparations. These results indicate that bis-phenolic compounds may be useful in studying the function of PM Ca2+-ATPase and regulation of intracellular Ca2+, and may have potential cardiovascular activities. Drug Dev. Res. 46:235–249, 1999.© 1999 Wiley-Liss, Inc.