Mutation of Phe-208 for Ile in monoamine oxidase (MAO) A and mutation of Ile-199 for Phe in MAO B inverted substrate specificity and inhibitor selectivity of the mutants towards the opposite form of the enzyme, MAO B and MAO A, respectively (Y. Tsugeno and A. Ito (1997) J. Biol. Chem., 272, 14033-14036). However, according to data presented by Tsugeno and Ito such point mutations did alter kinetic characteristics of oxidative deamination of some but not all substrates. These point mutations did not alter the secondary structure evaluated by four computer methods. Analysis of the efficacy of a few series of inhibitors suggests spatial differences of substrate binding sites of MAO A and MAO B. All these discrepancies indicate that one amino acid can be responsible for binding of some but not all substrates and inhibitors.