Modulation of the low affinity Ca2+-binding sites of skeletal muscle and blood platelets Ca2+-ATPase by nordihydroguaiaretic acid

The antioxidant nordihydroguaiaretic acid (NDGA) inhibited the different sa rco/endoplasmic reticulum Ca2+-ATPase isoforms found in skeletal muscle and blood platelets. For the sarcoplasmic reticulum, but not for the blood pla telets Ca2+-ATPase, the concentration of NDGA needed for half-maximal inhib ition was found to vary depending on the substrate used and its concentrati on in the assay medium. The phosphorylation of the sarcoplasmic reticulum C a2+-ATPase by ATP and by P-i were both inhibited by NDGA. In leaky vesicles , measurements of the ATP<->P-i exchange showed that NDGA increases the aff inity for Ca2+ of the E-2 conformation of the enzyme, which has low affinit y for Ca2+. The effects of NDGA on the Ca2+-ATPase were not reverted by the reducing agent dithiothreitol nor by the lipid-soluble antioxidant butylat ed hydroxytoluene.