Na⁺ ions counteract the Ca²⁺ -induced inhibition of cytochrome <i>c</i> oxidase activity of bovine COX.

<p>(<b>A</b>) Bovine COX (30 nM) in the basic medium with 0.4 M choline chloride and 15 µM oxidized cytochrome <i>c</i>. The reaction is initiated by addition of 15 µM ferrocytochrome <i>c</i> and oxidation of <i>c</i>²⁺ is followed in a dual-wavelength mode at 550 nm vs the 540 nm reference wavelength. Dashed line, control recording (no additions). Solid line, 50 mM NaCl and 200 µM CaCl₂ (100 µM excess over EGTA) are added where indicated. (<b>B</b>) Bovine COX (1.3 nM) in the basic medium with 0.4 M choline chloride and with no oxidized cytochrome <i>c</i>. Reaction is initiated by addition of 6 µM ferrocytochrome <i>c</i>. Other additions, as in (<b>A</b>).