Redox properties of the flavin cofactor of monoamine oxidases A and B and their relationship to the kinetic mechanism

This chapter discusses the redox properties of the flavin cofactor of monoamine oxidases (MAO) A and B and their relationship to the kinetic mechanism that they follow. The purified enzyme preparations used in this study were the human liver MAO A expressed in yeast and bovine liver MAO B. The redox potentials in the absence of substrate were determined by the method developed by Massey. The redox potentials of cysteinyl-FAD in unliganded MAO A and B have been determined. The spectral changes were observed in a mixture of MAO B and a reference dye, indigo disulfonate. Reduction of the dye is characterized by the disappearance of the peak at 610 nm observed for the oxidized dye and the appearance of an equally prominent peak at 370 nm for the reduced dye. These changes are superimposed on the decrease at 456 nm observed for the reduction of cysteinyl-FAD from the oxidized form to a semiquinone. The redox potentials of the cysteinyl-FAD in unliganded MAO A and B are close to that for free flavin. It has been concluded that the redox potential in an enzyme-substrate complex is positively shifted towards the potential of an amine substrate. This shift is different for each substrate. The values for the redox potentials for MAO in the presence of physiological substrates remain to be determined. The rate of reduction of the flavin by substrate correlates with the redox potential—that is, an increase in the potential of the flavin favors electron transfer from amine to flavin.