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