Acidity constants dinucleotides

Xanthine oxidase, which is capable of catalyzing the conversion of hypoxanthine and xanthine to uric acid, was first detected in 1882 by Horbaczewski (Hll), who noted that extracts of various tissues could catalyze the conversion of xanthine to uric acid. A similar enzyme was detected in milk (M15). These enzymes contain a flavin-adenine dinucleotide prosthetic group (C9). As a result of the essential nature of the flavin-adenine dinucleotide portion of the enzjmie, a striking parallelism was seen between the riboflavin content of the diet and the xanthine oxidase concentration in tissues of growing rats (DIO). The enzyme contains molybdenum. That the molybdenum is contained in a functionally important component has been demonstrated by several workers (G13, T5). Totter and his associates injected labeled molybdate into a cow, and then isolated the enzyme from the milk to show that the proportion between the molybdenum and flavin remained constant at a value of 0.5. Corran et al. (C9) postulated that the xanthine oxidase of milk is identical with the xanthine oxidase of liver, but the protein portions of the enzyme appear to differ. 

The transient absorption spectra similar to that of the ion-pair state of indole cation radical and flavin anion radical were also observed in D-amino acid oxidase (5), although the spectra were not so clear as those of flavodoxin. In D-amino acid oxidase, the coenzyme, flavin adenine dinucleotide (FAD), is wealtly fluorescent. The fluorescence lifetime was reported to be 40 ps (16), which became drastically shorter (less than 5 ps) when benzoate, a competitive inhibitor, was combined with the enzyme at FAD binding site (17). The dissociation constant of FAD was also marlcedly decreased by the binding of benzoate (17). These results suggest that interaction between isoalloxazine and the quencher became stronger as the inhibitor combined with the enzyme. Absorbance of the transient spectra of D-amino acid oxidase-benzoate complex was remarkably decreased. In this case both rate constants of formation and decay of the CT state could become much faster than those in the case of D-amino acid oxidase free from benzoate. 

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