The pH dependence of the inhibition of ascorbate oxidase by anions

Double-reciprocal plots of azide inhibition, with respect to ascorbate, of ascorbate oxidase indicate mixed-type inhibition at pH values above 6. This is in contrast to the simple competitive inhibition previously observed at pH 5.6. Linear replots of the slopes and intercepts of the double-reciprocal plots yield two inhibition constants. Both constants are pH dependent. Similar inhibition patterns are obtained with fluoride and thiocyanate. These results suggest the presence of two inhibitor binding sites, one of which is competitive with respect to ascorbate and the other uncompetitive.