Figure 5. Na⁺ ions counteract the Ca²⁺ -induced inhibition of cytochrome c oxidase activity of bovine COX.

(A) Bovine COX (30 nM) in the basic medium with 0.4 M choline chloride and 15 µM oxidized cytochrome c. The reaction is initiated by addition of 15 µM ferrocytochrome c and oxidation of c 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) Bovine COX (1.3 nM) in the basic medium with 0.4 M choline chloride and with no oxidized cytochrome c. Reaction is initiated by addition of 6 µM ferrocytochrome c. Other additions, as in (A).