Species differences in the interactions of the anticonvulsant milacemide and some analogues with monoamine oxidase-B.

Oxidation of the anticonvulsant drug milacemide [2-n-(pentylamino)acetamide] by monoamine oxidase-B (MAO-B) has been reported to be important in terminating its activity. Comparison of the oxidation of this compound by MAO-B preparations from ox and rat liver showed the former enzyme to have a significantly higher Km value towards this substrate. In keeping with this, the Ki values for milacemide acting as a competitive inhibitor of these enzymes showed it to have a lower affinity for ox liver MAO-B. Comparative studies on the time-dependent inhibition of the two enzymes also showed a lower sensitivity of that from the ox liver. Studies with a series of analogues involving replacement of pentylamino group of milacemide showed marked differences between the sensitivities of the two enzymes. The largest differences were shown by the compound 2(4-(3-chlorobenzoxy)phenethylamino)acetamide which gave IC50 values of 0.051 +/- 0.008 and 4.1 +/- 0.8 microM with the rat and ox enzymes, respectively, when activities were assayed without prior enzyme-inhibitor preincubation. When the enzyme and inhibitor were incubated for 60 min at 37 degrees before assay these values fell to 0.027 +/- 0.002 and 3.5 +/- 0.4 microM, respectively. These marked differences prompted a study of the inhibition of MAO-A and MAO-B from human liver and brain, mouse brain and rat brain as well as MAO-B from ox liver by milacemide and alpha-methylmilacemide. There were no significant differences in the sensitivities of any of the mitochondrial MAO-A preparations studied towards these compounds. However, MAO-B from human brain and liver mitochondrial resembled that from ox liver in being less sensitive to inhibition than the rat and mouse enzymes. Purification of the ox liver MAO-B did not significantly affect its interactions with milacemide and alpha-methylmilacemide. The marked species differences reported here raise questions concerning the validity of rodent model systems, that have frequently been used for assessing the in vivo and in vitro actions of milacemide and its analogues, for the situation in the human.