The Absorption Spectra of Iron-Flavoproteins

The reddish appearance of milk xanthine oxidase was first noted by Corran, Dewan, Gordon, and Green (l), who attributed this property to the presence of a second chromophoric group in addition to the flavin component. Morrell(2) suggested that the seemingly atypical absorption spectrum of this enzyme is caused by enhancement of the flavin absorption resulting from the mode of binding to the enzyme protein. Richert and Westerfeld (3) demonstrated the presence of iron in purified xanthine oxidase, but, concluded that the spectrum of the enzyme was not correlated with the presence of iron. The presence of an additional chromophore was assumed by Mackler, Mahler, and Green (4) as well as by Westerfeld, Richert, and Higgins (5). Since the isolation of milk xanthine oxidase, a number of other flavoproteins with similarly anomalous absorption spectra have been reported, including calf liver xanthine osidase (6), chicken liver xanthine dehydrogenase (7), rabbit liver aldehyde oxidase (8), and xanthine dehydrogenase from Clostridium cylindrosporum (9). Nonheme iron and molybdenum have been shown to be associated with each of these enzymes. Dihydroerotic dehydrogenase from Zymobacterium oroticum, which contains 1 atom of iron per flavin but no molybdenum, exhibits significant absorption at wave lengths longer than 500 rnp and greater absorption at 450 rnN than that expected from its flavin content (10). The mechanisms of action of milk xanthine oxidase, rabbit liver aldehyde oxidase, and dihydroorotic dehydrogenase have been under intensive investigation in this laboratory. In the course of these studies, evidence has been obtained which indicates that the anomalous absorption spectra of each of these various flavoproteins reflects a single, specific form of ironprotein binding.