Catecholamine binding by adrenal medullary protein can interfere with a sensitive radioenzymatic assay for norepinephrine.

Norepinephrine (NE) binds extensively to protein that copurifies with phenylethanolamine-N-methyltransferase (PNMT) prepared from bovine adrenal medulla. This binding interferes with a NE assay that employs PNMT to catalyze the transfer of a tritiated methyl group from S-adenosyl-L-methionine to the amine group of NE. It was discovered that the protein binding of endogenous NE is reversed by dialysis at pH 6.0. Preparations of PNMT intended for use in radioenzymatic assays should involve one or more purification steps at pH 6.0.