Mechanism of Inactivation of Monoamine Oxidase B by (Aminomethyl)trimethylsilane

The mechanism of inactivation of mitochondrial monoamine oxidase (MAO) by (aminomethyl)trimethylsilane (1) has been investigated. The carbon analogue of 1, namely, neopentylamine, is a substrate for MAO, suggesting that the silicon atom is responsible for the inactivation properties of 1, which exhibits all of the characteristics of a mechanism-based inactivator. The inactivated enzyme slowly releases the adduct and becomes reactivated in a time-and pH-dependent reaction. Inactivation by [1-³H]-1 results in the incorporation of 1.24 equiv of 3H into the enzyme and the release of [³H]formaldehyde. Inactivation of MAO by [1-²H₂]-1 exhibits a deuterium isotope effect of 2.3 on inactivation and releases both mono-and dideuterated formaldehyde. Inactivation by 1 in²H₂O gives mono-and undeuterated formaldehyde. [¹⁴C-methyl]-1 inactivates the enzyme with the incorporation of 3.29 equiv of radioactivity. The rate of release of ¹⁴C from [¹⁴C-methyl]-1-inactivated enzyme is approximately the same as the rate of release of ³H from [1-³H]-l-inactivated enzyme and the rate of return of enzyme activity. The flavin becomes reduced during inactivation but is reoxidized upon denaturation. After 22 h of dialysis 0.44 equiv of tritium remains bound to the oxidized enzyme. These results indicate that 1 undergoes oxidation by two different pathways, but only one leads to inactivation, namely, oxidation to the corresponding trimethylsilyl iminium ion (presumably by a one-electron mechanism) followed by attack of an active-site amino acid nucleophile at the imine carbon. A one-electron oxidation mechanism followed by desilylation is supported as the alternate pathway that does not lead to inactivation. The conclusions regarding the reaction of 1 with MAO are discussed in terms of the normal catalytic mechanism of the enzyme.