SPECIES-DIFFERENCES IN THE INTERACTIONS OF THE ANTICONVULSANT MILACEMIDE AND SOME ANALOGS WITH MONOAMINE OXIDASE-B

Oxidation of the anticonvulsant drug milacemide [2-n-(pentylamino)acet amide] by monoamine oxidase-B (MAO-B) has been reported to be importan t in terminating its activity. Comparison of the oxidation of this com pound by MAO-B preparations from ox and rat liver showed the former en zyme to have a significantly higher K-m value towards this substrate. In keeping with this, the K-i values for milacemide acting as a compet itive inhibitor of these enzymes showed it to have a lower affinity fo r ox liver MAO-B. Comparative studies on the time-dependent inhibition of the two enzymes also showed a lower sensitivity of that from the o x liver. Studies with a series of analogues involving replacement of p entylamino group of milacemide showed marked differences between the s ensitivities 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 mu M with the rat and ox enzymes, respectively, when activities were assayed without prior e nzyme-inhibitor preincubation. When the enzyme and inhibitor were incu bated for 60 min at 37 degrees before assay these values fell to 0.027 +/- 0.002 and 3.5 +/- 0.4 mu M, respectively. These marked difference s prompted a study of the inhibition of MAO-A and MAO-B from human liv er and brain, mouse brain and rat brain as well as MAO-B from ox liver by milacemide and alpha-methylmilacemide. There were no significant d ifferences in the sensitivities of any of the mitochondrial MAO-A prep arations studied towards these compounds. However, MAO-B from human br ain and liver mitochondria resembled that from ox liver in being less sensitive to inhibition than the rat and mouse enzymes. Purification o f the ox liver MAO-B did not significantly affect its interactions wit h milacemide and alpha-methylmilacemide. The marked species difference s reported here raise questions concerning the validity of rodent mode l systems, that have frequently been used for assessing the in vivo an d in vitro actions of milacemide and its analogues, for the situation in the human.