Desulfo enzymes

Desulfo enzymes can be regenerated by anaerobic treatment with sulfide plus dithionite. Cyanide-inactivated turkey liver xanthine dehydrogenase is also reactivated by selenide, suggesting that selenium may replace sulfur in the active centre. 

Deactivation of oxidized xanthine oxidase by cyanide produces thiocyanate and an inactive or desulfo form of the enzyme. The oxidized desulfo enzyme contains a dioxo-Mo(VI) center. Since it is generally believed that a terminal thio ligand on Mo is removed in the cyanolysis reaction, the interaction of oxo-thio-Mo(VI) complexes with CN is worthy of investigation. In general, reactions of thio-Mo( VI) complexes (typically thiomolybdates) with cyanide leads to the formation of SCN" by formal sulfur atom transfer and reduction of the metal, e.q., Eq. (25). 

The xanthine oxidase enzymes typically contain two different [Fe2S2] clusters and a flavin, plus the active site and the focus of this review, the Mo. The enzymes react with cyanide to liberate thiocyanate reduced inactive desulfo enzyme, which is missing an essential active-site sulfur ... 

However, achieving a full understanding of the system came slowly, largely because of complications caused by slow equilibration of the desulfo enzyme with the functional enzyme system. Thus, it was not until much later that the relative (Olson et al, 1974) or the absolute (Bray et al, 1975 Cammack et al, 1976) redox potentials for equation (2) could be estimated for functional and for desulfo xanthine oxidase. 

Other biochemical data also seemed consistent with this formulation of the structures. However, there are problems. One concern is the possibility, indeed the likelihood, that a sulfur atom of an -MoSH structure would be liberated when the solution was acidified. If this were the case, taking into account acid-labile sulfur from iron-sulfur centers, then the desulfo enzyme... 

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