Transformation of monoamine oxidase-B primary amine substrates into time-dependent inhibitors. Tertiary amine homologs of primary amine substrates

Abstract

A family of N-methylated and N,N-dimethylated alkyl and arylalkylamines was prepared and more than half of the analogues were shown to be time-dependent pseudo-first-order inhibitors of monoamine oxidase-B. Some of the time-dependent inactivators were reversible and others were irreversible with respect to prolonged dialysis following inactivation. Partition ratios ranged from zero to 11,000. These results are rationalized in terms of a combination of an inductive effect and a stereoelectronic effect as a result of hindered rotation of an active site covalent adduct. A molecular mechanics calculation indicates that there is at least 10 kcal/mol of torsional energy to be overcome in order for the enzyme adduct to be released. These findings show that tertiary amine homologues of primary amine substrates of monoamine oxidase are time-dependent inhibitors, and this should be useful in the design of new inactivators of this enzyme.

How are Biogenic Amines Metabolized by Monoamine Oxidases?

“…Upon deprotonation the system is stabilized by 10.1 kcal mol –1 to 1 SP3 , making the whole reaction energetically feasible and yielding the neutral trans ‐imine and the reduced flavin (FADH 2 ) as the final products (Figure 2 and Scheme ). It has to be strongly emphasized that the presence of the aforementioned acidic amine N–H bond enables the completion of MAO turnover and explains why many alkyl‐ and arylamines change from being MAO substrates to MAO inhibitors upon N , N ‐dimethylation 46. The fact that dopamine is converted into a neutral imine is significant, because this suggests that it will predominantly remain unprotonated in the hydrophobic active site, based on consideration of the p K a values of similar unconjugated imines, which are, as a rule, found to be below the physiological value of 7.4 (for example, the p K a value of Me 2 C=N–Me is 5.5),37 ensuring that the neutral product could go past the “aromatic cage” on its release from the active site.…”

What a Difference a Methyl Group Makes: The Selectivity of Monoamine Oxidase B Towards Histamine and N‐Methylhistamine

“…Following the initial hydride transfer,t he system relaxes to the corresponding intermediates, which are characterizedb y the formed semi-reduceda nionic flavin, FADH À ,a nd the cationic substrate (Figure 4a nd Figure 5). This is why the adduct formation rationalizes why many alkyl-and arylamines change frombeing MAO substrates to irreversible MAO inhibitors upon N,N-dimethylation, [38] making the adduct formation with dopamine fully justified. It is important to emphasize that as ignificant differencec omparedw ith dopamine degradation is that, in the intermediate, there is no adduct formation between flavin and either HIS or NMH as it was demonstrated to occur with dopamine( Scheme 3), in which the N5(flavin)ÀC(a)a dduct was rather stable (DG form = À27.7 kcal mol À1 )a nd formed spontaneously following the H À transfer.…”

“…[9] In NMH,t his process is accompanied with the activation free energy of 4.8 kcal mol À1 (n imag = 792i cm À1 ), which is slightly reduced in HIS to 2.8 kcal mol À1 (n imag = 834i cm À1 ). [33,37,38] Finally,i ti sv ery important to put the obtained activation free energies in the right perspective.A tf irst sight, the calculated DG°(NMH) = 20.4 kcal mol À1 and DG°(HIS) = 23.0 kcal mol À1 appear skewed from experimental DG°E XP (NMH) = 17.8 kcal mol À1 and DG°(HIS) = 19.2 kcal mol À1 ,w hich are derived from the measured k cat (NMH) = 35 min À1 and k cat (HIS) = 3.5 min À1 values. [13] The low barrier of the second step is easily rationalized if one considers the corresponding pK a values of the interacting sites.…”

The Use of Multiscale Molecular Simulations in Understanding a Relationship between the Structure and Function of Biological Systems of the Brain: The Application to Monoamine Oxidase Enzymes

“…Upon deprotonation, the system is stabilized by 10.1 kcal mol −1 to 2 SP 3 , making the whole reaction energetically feasible and yielding the neutral trans-imine and the fully reduced flavin (FADH 2 ) as final products (Figures 3 , 4 ). It has to be strongly emphasized that the presence of the acidic N–H bond enables the completion of MAO turnover and explains why many alkyl- and arylamines change from being MAO substrates to MAO inhibitors upon N , N -dimethylation (Ding et al, 1993 ). The fact that dopamine is converted into a neutral imine is significant, because this suggests it will predominantly remain unprotonated in the hydrophobic active site, based on consideration of the p K a values of similar unconjugated imines, which are, as a rule, found to be below the physiological value of 7.4 (for example, the p K a value of Me 2 C=N−Me is 5.5) (Smith and March, 2001 ), ensuring that the neutral product could go past the “aromatic cage” on its release from the active site.…”