Effect of alpha-methylation on inactivation of monoamine oxidase by N-cyclopropylbenzylamine.

Monoamine oxidase (MAO) was shown previously [Silverman, R. B., & Hoffman, S. J. (1980) J. Am. Chem. Soc. 102, 7126-7128] to catalyze the oxidation of N-cyclopropylbenzylamine (N-CBA) at two sites on the molecule. Oxidation at the benzyl methylene gave benzaldehyde and cyclopropylamine; oxidation of the cyclopropyl group, which involved cyclopropyl ring cleavage, led to inactivation of the enzyme. In this paper it is shown that methylation of the benzyl methylene dramatically alters this partition ratio in favor of enzyme inactivation. Contrary to a previous report [Alles, G., & Heegaard, E. V. (1943) J. Biol. Chem. 147, 487-503], it is shown here that alpha-methylbenzylamine is a substrate for MAO; consequently, N-cyclopropyl-alpha-methylbenzylamine (N-C alpha MBA) is a good candidate for mechanism-based inactivation. N-Cyclopropyl[7-14C]benzylamine, N-cyclopropyl-alpha-methyl[phenyl-14C]benzylamine, N-[1-3H]-cyclopropylbenzylamine, and N-[1-3H]cyclopropyl-alpha-methylbenzylamine are synthesized, and product formation following MAO inactivation is quantified. The results obtained with these compounds indicate that with N-C alpha MBA, alpha-methylbenzyl oxidation (which produces acetophenone and cyclopropylamine) is only 1% that of cyclopropyl oxidation (which gives enzyme inactivation), whereas with N-CBA the amount of oxidation at the corresponding sites is equal. It also is shown that the Ki values for (R)-(+)- and (S)-(-)-alpha-methylbenzylamine are similar, suggesting that dimethylation of N-CBA should not interfere with binding to MAO.(ABSTRACT TRUNCATED AT 250 WORDS)