The Content and Possible Catalytic Significance of Labile Sulfide in Some Metalloflavoproteins

Absorptionspectra of metalloflavoproteins differ from those of other flavoproteins by having absorption at wave lengths longer than 500 rnp, enhanced absorption in the 450 rnp region, and a less distinct or even totally absent absorption maximum in the 370 m/l region.As pointed out by Rajagopalan and Handler (l), deviations from the absorption spectra of free flavins are correlated to some extent with the metal content of the enzyme.Thus the differences are less for dihydroorotic dehydrogenase, containing 1 mole of iron per mole of flavin (24), than for reduced nicotinamide adenine dinucleotide-cytochrome c reductase, containing 2 moles of iron per mole of flavin ( 5).Even greater deviation is found for succinic dehydrogenase (6) and xanthine oxidase (7-13), both of which contain 4 or more moles of iron, and the latter also 1 mole of molybdenum per mole flavin.The most extensive deviation of all is found for reduced nicotinamide adenine dinucleotide dehydrogenase, in which 17 to 18 moles of iron per mole of flavin arc present (14, 15).It is not surprising that such direct correlation between metal content and spectral characteri&ics, together with some results from studies with model systems (16-l@, should have led to the suggestion that some form of interaction between the metal and the prosthetic group might be responsible for the noted deviations.However, further work with model systems (19,20) made it increasingly doubtful that such an interaction could provide a complete explanation (21).The presence of labile sulfide in succinic dehydrogenase, together with the effect of urea and mercurials on the spectrum and rate of reaction with iron chelators (22, 23), first suggested that a more complex arrangement might be involved.Indirect support for this possibility was provided by the results of investigations of the ferredoxins.Addit,ion of o-phenanthroline or sulfhydryl reagents to these iron sulfide proteins caused absorption changes associated with losses of iron, labile sulfide, and activity (24, 25).In this paper evidence is presented which suggests that in liver santhine oxidase the labile sulfide is linked to the protein through ferric iron and another sulfur atom.