The molybdoenzymes xanthine oxidase and aldehyde oxidase contain fast- and slow-DTNB reacting sulphydryl groups.

The reactivities with an excess of 5-5'-dithiobis (2-nitrobenzoic) acid (DTNB) of sulphydryl residues present in xanthine oxidase and aldehyde oxidase were studied and compared. The results show that two classes of sulphydryl groups with quite different reactivities exist in both enzymes either native or denatured. Some of the available sulphydryl residues thus react instantaneously with the DTNB, whereas the others react very slowly following pseudo-first-order kinetics. The number of sulphydryl residues of each class and the rate constant of slowly reacting groups are, respectively, 1.7 and 0.8 in native xanthine oxidase and 1.6 and 1.7 in native aldehyde oxidase. In denatured enzymes, the number of fast- and slow-reacting sulphydryl residues obtained are, respectively, 13.9 and 7.9 in xanthine oxidase and 5.7 and 5.4 in aldehyde oxidase. Analogously, the rate constant for the slowly reacting groups is similar for the two native enzymes, but in denatured aldehyde oxidase it is double that of denatured xanthine oxidase.