Radioenzymic measurement of norepinephrine, epinephrine, and dopamine: stability, enzymic activity, and sensitivity.

We investigated factors affecting sensitivity and reproducibility of radioenzymic (catechol-O-methyltransferase) measurement of plasma catecholamines. There was no apparent deterioration of catecholamine during storage at -20 degrees C for 240 days. Accuracy and sensitivity of the assay depended mostly on the method of transferase preparation; its preparation by adjusting the pH to 6.8 before fractionation with ammonium sulfate resulted in an unsuitable enzyme with a relatively high background-radioactivity count, particularly for neorepinephrine and dopamine.