Comparison of o-octopamine and related phenylethanolamines as substrates for norepinephrine N-methyltransferase

o-Octopamine was a substrate for rabbit adrenal norepinephrine N-methyltransferase with lower affinity than the position isomers m-octopamine and p-octopamine. The order of substrate affinity for monochloro or monohydroxy phenylethanolamines was para greater than meta greater than ortho. With phenylethanolamine and ring-hydroxylated phenylethanolamines, primary amines were better substrates than were N-methyl secondary amines. N-Methylation might be a metabolic pathway for o-octopamine, reported to occur in mammalian tissues.