Mechanism of inhibition of the (Ca2(+)-Mg2+)-ATPase by nonylphenol.

The effects of nonylphenol and 3,5-dibutyl-4-hydroxytoluene (BHT) on the activity of the (Ca2(+)-Mg2+)-ATPase of skeletal muscle sarcoplasmic reticulum have been studied. At high concentrations, both inhibit the ATPase activity of the ATPase either in native lipid or in bilayers of dioleoylphosphatidylcholine but, at low concentrations, an increase in ATPase activity is observed, particularly for the ATPase reconstituted into dimyristoleoylphosphatidylcholine. Neither nonylphenol nor BHT binds at the lipid-protein interface of the ATPase. Nonylphenol decreases the effective equilibrium constant for phosphorylation of the ATPase by Pi probably through an increase in the effective rate of dephosphorylation of the phosphorylated ATPase. It also decreases the effective rate of the E2-Ca2E1 transition and increases the effective equilibrium constant E2/E1 for the ATPase. Inhibition of ATPase activity follows from the slowing of the E2-E1 transition despite increases in effective rates for dephosphorylation and for the transport step, Ca2E1P-E2P. Since nonylphenol has been shown to affect equilibrium constants for various steps in the reaction pathway of the ATPase, inhibition of activity of the ATPase cannot follow from effects on the fluidity (viscosity) of the membrane, since fluidity alone cannot affect equilibrium properties of the system.