Stereochemical aspects of phenylethanolamine analogues as substrates of phenylethanolamine N-methyltransferase.

Phenylethylamines and phenylethanolamines represent two major classes of ligands for the epinephrine synthesizing enzyme, phenylethanolamine N-methyltransferase (PNMT;EC 2.1.1.28). Phenylethylamines are usually competitive inhibitors and the isomers with the relative configuration as in (2S)-amphetamine (1) and (2S)-2-aminotetralin (3) are better inhibitors than their enantiomers. Phenylethanolamines are usually substrates of PNMT and the enzyme prefers the 1R isomers, such as (1R)-phenylethanolamine (5), in this class. Optically active norephedrines (7 and 8), norpseudoephedrines (9 and 10), and 2-amino-1-tetralols (13-16) were used to study the stereochemical requirements of phenylethanolamines for PNMT active site binding. Although the norephedrines (7 and 8) and the norpseudoephedrines (9 and 10) were poorer ligands for PNMT than were the 2-amino-1-tetralols (13-16), (1R,2S)-(-)-norephedrine (7) showed some activity as a PNMT substrate (Km = 1310 microM, Vmax = 0.22, 100 x Vmax/Km = 0.017). In the 2-amino-1-tetralols (13-16), the isomers with the 2S configuration (13 and 15) showed higher affinity to PNMT (13, Km = 4.5 microM; 15, Ki = 4.6 microM) and those with the 1R configuration (13 and 16) were substrates for the PNMT-catalyzed methyl transfer (13, Km = 4.5 microM, Vmax = 0.16, 100 x Vmax/Km = 3.6; 16, Km = 195 microM, Vmax = 0.12, 100 x Vmax/Km = 0.062); the combination of 1R and 2S configurations, such as in (1R,2S)-2-amino-1-tetralol (13), was required for a good substrate. These stereochemical requirements derived from the norephedrines (7 and 8), the norpseudoephedrines (9 and 10), and the 2-amino-1-tetralols (13-16) complement those for phenylethylamines (1-4) and for phenylethanolamines (5 and 6) and strongly suggest that phenylethylamine inhibitors bind to PNMT in the same orientation as do phenylethanolamine substrates.