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A New Strategy for the Design of Monoamine Oxidase Inactivators. Exploratory Studies with Tertiary Allylic and Propargylic Amino Alcohols
A new strategy for the design of monoamine oxidase (MAO) inhibitors is proposed. The strategy is based on the premise that tertiary-amine containing MAO-inactivators which operate by alkylation of active site nucleophiles are activated in situ by single electron transfer (SET) to the MAO-flavin cofactor to form aminium cation radicals which undergo secondary fragmentation reactions to produce reactive electrophiles. The purpose of the current work was to assess the feasibility and applicability of this proposal for the design of new families of MAO-inactivators. Based on the documented retro-aldol type fragmentation reactivity of α-amino-alcohol cation radicals, tertiary α-allylic and -propargylic β-amino-alcohols were expected to serve as precursors of conjugated ketones in SET-promoted processes. Evidence supporting this hypothesis was gained from studies of model SET-photoreactions of members of this amino-alcohol family with 3-methyllumiflavin (3MLF). The efficient production of 4a- and 4a,5-flavin adducts in these excited-state reactions demonstrates that aminium radicals, arising by SET-oxidation of tertiary α-allylic and -propargylic β-aminoalcohols, fragment to generate α,β-unsaturated ketones which react rapidly with the simultaneously formed 3MLF-hydroflavin anion. The second feature of the MAO-inactivator design strategy pathway was tested by examining reactions of the MAOs with substances which contain electrophilic, conjungated enone and ynone moieties tethered to amine functions to ensure delivery to the enzyme active sites. The covalent modification of active site cysteine thiol residues by the unsaturated ketone groups in these substances was confirmed by demonstrating that they serve as active site-directed, time-dependent, nonredox based, inactivators of MAO-A and MAO-B. In the key test of the feasibility of the new MAO-inactivator design strategy, it was shown that selected tertiary α-allylic and -propargylic β-amino-alcohols undergo redox reactions in the MAO-A active site which result in inactivation of the enzyme via covalent modification of a single cysteine residue. The experimental results which support the conclusions stated above are presented and discussed in this paper.
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