The effects of inhibitor mixtures and the specific effects of different anions on the oxidase activity of caeruloplasmin.

1. The interpretation of the effects of mixtures of inhibitors on enzymes is considered. 2. The effects of inhibitor mixtures on caeruloplasmin were determined. 3. Fluoride, chloride and cyanate inhibit at one type of site (alpha), whereas bromide and iodide inhibit at another type (beta) present in the same enzyme intermediate. 4. Effects of inhibitor mixtures containing azide or cyanide are consistent with previous indications (Speyer & Curzon, 1968) that these ligands form inhibited complexes with different enzyme intermediates. 5. Isobols of halides or of cyanate with azide indicate that azide inhibits caeruloplasmin by bridging two alpha sites, these being reduced copper atoms. 6. Iodide and cyanate give hyperbolic plots of 1/v against [I]. 7. It is suggested that in the cyanate-inhibited complex the inhibitor binds to a reduced copper atom (alpha site) but that binding of cyanate at another copper atom is sterically prevented. It is suggested that the less bulky alpha-site inhibitors, fluoride and chloride, cause complete inhibition by binding to both of these copper atoms, which can also be bridged by a single azide group. 8. Each halide shows a pattern of effects on caeruloplasmin that is qualitatively distinct from that of other halides.