Normetanephrine, an N-methylatable phenylethanolamine, is converted to its N-methylated, tritiated derivative, metanephrine, utilizing phenylethanolamine-N-methyltransferase as enzyme and tritiated S-adenosyl-L-methionine as methyl donor. The formed tritiated derivative is selectively extracted in organic solvents and separated from other N-methylatable phenylethanolamines by thin layer chromatography. This assay could be used for measurement of normetanephrine in biological systems of patients with hypertension for detection of pheochromocytoma.

An improvement in the method for assaying normetanephrine and octopamine. A procedure is used in which the phenylethanolamine in a sample is enzymatically transmethylated in an incubation mixture of the phenylethanolamine, a compound containing a transferable tritiated methyl group and a transfer enzyme, to form a tritiated N-methyl derivative, and the radioactivity of the derivative is measured. In accordance with the present improvement, incubation of the mixture is conducted at a pH of at least 8.6, preferably in the range of 8.8-9.3. Prior to incubation, as appropriate, the sample may be deproteinized and the phenylethanolamine concentrated.

A method of measuring the individual response to antidepressant drug therapy on the transport inhibition of monoamine neurotransmitters involves in vitro monitoring of radiolabeled monoamine neurotransmitter transport into cells transfected with transport proteins similar to those on neural cells of the individual being studied. The transport occurs in unbuffered serum of the individual who is undergoing or will later undergo pharmaceutical treatment for depression or other neuropsychiatric disorders. The use of buffers is avoided so that the sensitive balance of bound/free drug within the individuals serum is not disrupted prior to or during testing.