A key factor for flavoenzyme activity is the reduction potential of the bound flavin. The reduction potentials of protein-bound flavins span approximately a 500-mV range consistent with flavoenzymes having critical roles in metabolism and a variety of biological processes. Redox potentials of flavoenzymes have traditionally been determined using an electrode-based system with either direct or indirect electrochemical coupling between the protein and the working electrode. An electrode independent method, however, is also now commonly used and involves calculating the unknown flavin reduction potential of the protein from the known reduction potential of a reference or indicator dye. Here, the "classic" potentiometric method and the xanthine/xanthine oxidase methods are described. Both methods rely on equilibrium between protein-bound flavin and redox dyes. The potentiometric method measures the equilibrated redox potential of the protein-dye mixture whereas the xanthine/xanthine oxidase technique relies on slow continuous enzymatic reduction to maintain a constant equilibrium between the protein and the dyes. Because electrochemical equipment is not required, the xanthine/xanthine oxidase method is more accessible and convenient for researchers seeking to determine reduction potentials of flavoproteins or other biological redox centers such as hemes. The xanthine/xanthine oxidase method has been used to determine flavin reduction potentials from +132 to -417mV, demonstrating it is suitable for characterizing the redox properties of most flavoproteins.
Keywords: Flavin; Flavoenzyme; Potentiometric titration; Reduction potential; Xanthine; Xanthine oxidase.
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