The inhibition of caeruloplasmin by cyanide.

1. The reversible inhibition of the oxidase activity of caeruloplasmin by cyanide was investigated. 2. The kinetics are unusual, being competitive but with the inhibited complex formed only during cycling. 3. Inhibitory concentrations of cyanide are comparable with that of caeruloplasmin. 4. One azide group completely inhibits a caeruloplasmin molecule but two cyanide groups are required. 5. The results suggest that azide binds to a half-reduced or fully reduced conformational isomer of the enzyme whereas cyanide binds to completely reoxidized isomers, and that inhibited complexes contain ligand bridges between copper atoms.