The action of cyanide and other respiratory inhibitors on xanthine oxidase

It was stated by Dixon and Thurlow (1925) that xanthine oxidase is not inhibited by cyanide. They found that concentrations of cyanide up to M/100 did not produce any inhibition of either the uptake of oxygen or the reduction of methylene blue by hypoxanthine in presence of the enzyme. Some inhibition was, however, observed with higher concentrations of cyanide. During the course of some other work on xanthine oxidase, Dixon and Lemberg, using the methylene blue technique, observed in 1931 (unpublished) that even small concentrations of cyanide completely inactivated the oxidase, provided that it was incubated with the cyanide for some time at 37° before testing. Similar results were later obtained by Keilin (unpublished) for O2 uptake. A reinvestigation of the action of cyanide on the xanthine oxidase was therefore desirable, and the results obtained are described in the present paper. It has been found that the action of cyanide on this oxidase shows a number of peculiar features which distinguish it from the other cases of cyanide inhibition hitherto known.