The nature of the inhibition of rat liver monoamine oxidase types A and B by the acetylenic inhibitors clorgyline, l-deprenyl and pargyline

The kinetics of inhibition of rat liver mitochondrial Monoamine Oxidase by clorgyline, l-deprenyl and pargyline are consistent with a mechanism whereby a reversible interaction between the inhibitor and the Enzyme active site under conditions of thermodynamic equilibrium is followed by a time-dependent formation of the covalently-bound enzyme-inhibitor adduct. The Ki value for the reversible interaction between clorgyline and Monoamine Oxidase A is about 1000 times lower than that towards the B-form of the Enzyme, and this difference is sufficient to account for most, but not all, of the selectivity of the inhibition caused by this compound. The Ki value of the Monoamine Oxidase B selective inhibitor l-deprenyl towards that form of the Enzyme is only about 40-fold lower than that towards the A-form. However, in this case, the rate of formation of the irreversible adduct is considerably faster for the B-form than for the A-form and this makes a major contribution to the selectivity of this compound. Pargyline shows a Ki value towards Monoamine Oxidase B that is only 8 times lower than that towards the A-form and in this case the rates of formation of the enzyme-inhibitor adducts are similar. The significance of these results are discussed in terms of the selective inhibition of Monoamine Oxidase.