Previous work in this laboratory1-5 has shown that when epinephrine labeled with C14 in the N-methyl group of the side-chain was infused, approximately one-third of the radioactivity was recovered in the urine. These data suggested the following conclusions: (1) Approximately two-thirds of the infused epinephrine lose the N-methyl group of the side-chain via the monoamine oxidase enzyme system (MAO), and (2) approximately one-third of the infused epinephrine retains the N-methyl group, of which 1%-5% represents epinephrine and its conjugate and the remainder represents metanephrine and its conjugate.
If a patient under therapy with a MAO inhibitor is infused with N-methyl-labeled epinephrine, more than one-third of the infused radioactivity should be recovered in the urine if the MAO is inhibited. The following information is thus obtained: 1. One may determine whether or not the compound is entering the blood stream in amounts sufficient
RESNICK O, HAGOPIAN M, HOAGLAND H, FREEMAN H. An in Vivo Test for Measuring Monoamine Oxidase Inhibition in Human Subjects. AMA Arch Gen Psychiatry. 1960;2(4):459-461. doi:10.1001/archpsyc.1960.03590100099011