Substrates but Not Inhibitors Alter the Redox Potentials of Monoamine Oxidases

Abstract

The midpoint potentials for the reduction of the cysteinyl-flavin adenine dinucleotide (FAD) in monoamine oxidases (MAO) A and B in the absence and presence of ligands have been determined. Both MAO A and MAO B can be reduced chemically in two steps, the first generating a semiquinone spectrum and the second the spectrum of fully reduced FAD, each of which requires two electron equivalents. The midpoint potentials for the oxidized/semiquinone and semiquinone/reduced couples were -159+/-4 mV and -262+/-3 mV for MAO A and -167+/-4 mV and -275+/-3 mV for MAO B. After modification with a thiol reagent, direct reduction from the oxidized to fully reduced form was observed with no semiquinone and without change in the overall midpoint potential. In the presence of substrate, no semiquinone was formed, but the midpoint potential for full reduction of the flavin was positively shifted by up to 500 mV, depending on the substrate. This shift in potential could permit a more thermodynamically favorable transfer of electrons from the amine substrates to oxygen. In contrast, stable products and inhibitors did not cause a shift in potential and did not prevent the formation of semiquinone.

Kinetics, mechanism, and inhibition of monoamine oxidase

“…For the cofactor, the redox potential in MAO was determined by reductive titration with dithionite in the presence of mediator dyes. For human MAO A, the value for the first electron reduction to the anionic semiquinone was -159 mV and for the second redox couple from semiquinone to the quinone at pH 7.4 was -262 mV and similar values were found for bovine MAO B (-167 and -275 mV) (Sablin and Ramsay 2001), consistent with the accumulation of semiquinone to about 30% of the total flavin during dithionite reduction. For cloned human MAO B, the one-electron reduction potentials were found to be much closer and slightly positive at +0.043 V for the first electron reduction and +0.037 V for the semiquinonequinone couple (Edmondson et al 2007).…”

Mechanistic Studies of the Flavoenzyme Tryptophan 2-Monooxygenase: Deuterium and 15N Kinetic Isotope Effects on Alanine Oxidation by an l-Amino Acid Oxidase

“…Moreover, energetic arguments have previously been raised against formation of the initial radical pair. Walker and Edmondson (39) argue that flavoenzymes, whose reduction potentials are typically less than 0.2 V vs. SHE 8 , are unlikely to catalyze the one electron oxidation of primary amines, which have estimated reduction potentials well above 1 V vs. SHE 9 . In light of these arguments, the simplest conclusion is that TMO does not utilize any of the currently proposed single electron transfer mechanisms.…”

Structures and mechanism of the monoamine oxidase family

“…Furthermore, the thermodynamic barrier to single electron transfer is quite high, with the high redox potential of amines ( E m = + 1.5 V) making them unlikely to be oxidized by the flavin cofactor ( E m = −0.2 to 0 V), and there is no alternate oxidant strong enough to abstract an electron from the amine substrate (31). Although substrate binding might increase the redox potential of the flavin cofactor in MAO A and B (40), the difference in potentials still remains unfavorable.…”