Suicide Inhibition of Monoamine Oxidase from Different Species by Milacemide

The kinetic parameters for the interactions of the anticonvulsant compound milacemide (2-n-pentylaminoacetamide) with MAO-B from different species have been determined. The data presented here supports the proposed mechanism by which milacemide acts as both a substrate and inhibitor of MAO-B (as a suicide substrate) and show milacemide to be abetter substrate than it is an inhibitor for MAO-B from all species, with partition ratios for rat, human and ox liver mitochondrial MAO-B of 12,615; 19,032 and 60,874, mol of product per mol of enzyme inactivated, respectively. In accordance with its higher relative specific activity towards milacemide and the slower rate of inhibition, the partition ratio for ox liver MAO-B is considerably higher than that of the enzyme from rat liver. The partition ratio for human liver mitochondria is close to that of rat liver mitochondria in agreement with the similarity for the half lives (t1/2) of these two preparations whichwere much lower than their respective (t1/2 ) values for the ox preparation. The values obtained for the apparent Km value (K) (mM) for the inhibition reaction and the inactivation constant (k in) (min-1) for the inhibition were (K ) = 0.315, 0.330 and 0.672, (k in) = 0.024 , 0.020 and 0.009 for rat liver mitochondrial MAO-B, rat liver mitochondrial outer-membrane MAO-B and ox iiver mitochondrial MAO-B, respectively. At low milacemide concentrations the relative affinities of, the rat liver mitochondrial MAO-B and ox liver mitochondrial MAO-B for inhibition by milacemide will be given by their relative values of k in / K (the inhibition specificity constant), this shows milacemide to be a better inhibitor of the rat liver enzyme. The k cat /k m values confirmed the fact that milacemide is a better substrate for rat liver MAO-B than for ox liver MAO-B and human liver MAO-B. Though the k in values confirmed that milacemide is a much better inhibitor for rat and human liver MAO-B than ox liver MAO-B