Syntheses of amino nitrones. Potential intramolecular traps for radical intermediates in monoamine oxidase-catalyzed reactions

• Published in: Bioorganic & medicinal chemistry 1998-12, Vol.6 (12), p.2405-2419
• Main Authors: Zhong, Boyu, Lu, Xingliang, Silverman, Richard B.
• Format: Article
• Language: English
• Subjects: Amines - chemical synthesis; Amines - chemistry; Amines - pharmacology; amino nitrones; Animals; Cattle; enzyme mechanism; Enzyme Stability; Free Radicals; Hydrogen-Ion Concentration; Indicators and Reagents; Kinetics; Liver - enzymology; Molecular Structure; Monoamine oxidase; Monoamine Oxidase - chemistry; Monoamine Oxidase - metabolism; Monoamine Oxidase Inhibitors - chemical synthesis; Monoamine Oxidase Inhibitors - chemistry; Monoamine Oxidase Inhibitors - pharmacology; Nitrogen Oxides - chemical synthesis; Nitrogen Oxides - chemistry; Nitrogen Oxides - pharmacology; radical intermediate; Structure-Activity Relationship
• ISSN: 0968-0896; 1464-3391
• DOI: 10.1016/S0968-0896(98)80016-3

Monoamine oxidase (MAO) is a flavin-dependent enzyme that catalyzes the oxidative deamination of a variety of amine neurotransmitters and toxic amines. Although there have been several studies that support the intermediacy of an amine radical cation and an α-radical during enzyme catalysis, there is no direct, i.e. EPR, evidence for these species as they are formed. Amino nitrones have been designed which, upon radical formation would produce an intermediate that is a resonance structure of the corresponding nitroxyl radical, which should be observable by EPR spectroscopy. Syntheses of seven different amino nitrones, three acyclic, and four cyclic analogues were attempted. The protected amino nitrones were stable, but all three of the acyclic amino nitrones were unstable. One of the cyclic analogues was very stable ( 39), one was stable only in organic solvents ( 40), one was stable only in aqueous medium below pH 6.5 ( 41), and the other ( 42) was stable for just a short time at room temperature, decomposing to a stable free radical. None of these analogues produced a MAO-catalyzed radical, yet 41 is a poor substrate ( K m = 0.2 mM; k cat = 0.034 min −1) and 39 is a mixed inhibitor ( K i = 26.5 mM). Although this approach does not appear to be applicable to amino nitrones, it should be a valuable approach for other enzymes where radical intermediates are suspected and nonamine nitrones can be utilized. Syntheses of seven different amino nitrones, three acyclic, and four cyclic analogues were attempted. The acyclic analogues were unstable. One of the cyclic analogues was very stable, one stable only in organic solvents, and one stable below pH 6.5. None was found useful to detect radical intermediates in monoamine oxidase, but the approach should be viable for use with other enzymes.