Inhibition of monoamine oxidases A and B by simple isoquinoline alkaloids: Racemic and optically active 1,2,3,4-tetrahydro-, 3,4-dihydro-, and fully aromatic isoquinolines

Article abstract of DOI:10.1021/jm00163a025

A series of 1,2,3,4-tetrahydro-, 3,4-dihydro-, and fully aromatic isoquinolines were tested as substrates and/or inactivators of highly purified human monoamine oxidase A and B (MAO A and B). None were found to be a substrate for either enzyme, but many of these isoquinolines could selectively inhibit either MAO A or B. Stereoselective competitive inhibition of MAO A was found with the R enantiomer of all the stereoisomers tested, including salsolinol (K(i) = 31 μM), salsoline (K(i) = 77 μM), salsolidine (K(i) = 6 μM), and carnegine (K(i) = 2 μM). As a class, the 3,4-dihydro-isoquinolines were the most potent inhibitors tested (K(i) = 2-10 μM), and the fully aromatic isoquinolines had intermediate activity (K(i) = 17-130 μM) against MAO A. In contrast, only a few of these compounds markedly inhibited MAO B. 1,2,3,4-Tetrahydroisoquinoline, its 2-methyl derivative, and o-methylcorypalline gave apparent K(i) values of 15, 1, and 29 μM, respectively, and two 3,4-dihydroisoquinolines (compounds 22 and 25) showed substantial inhibition of MAO B (K(i) = 76 and 15 μM, respectively). These results support the concept that the topography of the inhibitor binding site differs in MAO A and B.