Conformationally restricted and conformationally defined tyramine analogues as inhibitors of phenylethanolamine N-methyltransferase.

In a search for a selective inhibitor for the epinephrine synthesizing enzyme phenylethanolamine N-methyltransferase (PNMT; EC 2.1.1.28), phenolic 2-aminotetralins (12-15 as conformationally restricted analogues of tyramine) and phenolic benzobicyclo[3.2.1]octylamines (22-24 as conformationally defined analogues of tyramine) were used to gain information about the binding interactions of the catecholic hydroxyl groups in the natural substrate norepinephrine at the active site of PNMT. In addition, these analogues provided information about the effects of conformational flexibility on active-site interaction of the aminoethyl side chain in phenolic phenylethylamines that may aid in learning the manner in which norepinephrine binds at the active site of PNMT. Analogues 22-24 were synthesized by a nine-step sequence, in which a Friedel-Crafts type intramolecular cyclization was the key step in the construction of the benzobicyclo[3.2.1]octane skeleton. p-Tyramine (10, Ki = 294 microM) was more potent than phenylethylamine (1, Ki = 854 microM) but m-tyramine (9, Ki = 1250 microM) was less potent than phenylethylamine as an inhibitor of PNMT. Similarly, in the conformationally restricted and conformationally defined tyramine analogues (12-15 and 22-24, respectively), the analogues with the p-tyramine moiety (14, Ki = 4.7 microM; 23, Ki = 111 microM) bind to PNMT better than do the corresponding unsubstituted compounds (16, Ki = 6.8 microM; 25, Ki = 206 microM) while the analogues with the m-tyramine moiety (13, 15, 22, and 24) have a lower binding affinity than do 16 and 25. The greatly enhanced activity of the phenolic 2-aminotetralins (12-15) compared with m- and p-tyramine (9 and 10, respectively) is likely due to the restriction of the side-chain conformation. The conformationally defined analogues 22-24 were less active than the conformationally restricted ones, 12-15, although the low-energy half-chair conformation of 2-aminotetralin is defined in 22-24. The reduced activity of 22-24 compared with the activity of 12-15 is probably due to the steric hindrance from the extra bridging atoms in binding to PNMT. The interaction of the p-hydroxyl group of the tyramine moiety may involve hydrogen bonding since the corresponding methyl ethers show a greatly reduced affinity for the active site of PNMT (Ki = 34 and 389 microM for methoxy analogues 28 and 35, compared to Ki = 4.7 and 111 microM for the corresponding phenolic analogues 14 and 23).