Comparison of Human Serum Dopamine-β-Hydroxylase Levels by Radioimmunoassay and Enzymatic Assay

Human serum dopamine-beta-hydroxylase (3,4-dihydroxyphenylethylamine, ascorbate:oxygen oxidoreductase, EC 1.14.17.1) has been measured in a normal adult population by both enzymatic assay and by radioimmunoassay. The levels of enzymatic activity varied over a very wide range from 0.6 units/ml to 85 units/ml. In contrast, the amount of total dopamine-beta-hydroxylase protein present in these samples, as measured by radioimmunoassay, varied only over a 3-fold range: from 11.7 mug/ml to 34.4 mug/ml. From an analysis of the purified enzyme, the percentage of serum dopamine-beta-hydroxylase that was catalytically active was estimated to be small and variable.