Comparison of human serum dopamine β hydroxylase levels by radioimmunoassay and enzymatic assay

Human serum dopamine β hydroxylase (3,4 dihydroxyphenylethylamine, ascorbate:oxygen oxidoreductase, EC 1.14.17.1) was 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 β hydroxylase protein present in these samples, as measured by radioimmunoassay, varied only over a 3 fold range: from 11.7 μg/ml to 34.4 μg/ml. From an analysis of the purified enzyme, the percentage of serum dopamine β hydroxylase that was catalytically active was estimated to be small and variable.