Human Serum Dopamine-0-Hydroxylase

Serum dopamine-/3-hydroxylase activity varied from 3 to 581 units/ml in a group of 168 untreated adults (90 normals, 78 with borderline or overt hypertension). Levels did not correlate with blood pressure or age, but mean values were significantly lower in blacks than they were in whites. Subjects with exceptionally low levels of serum dopamine-/3-hydroxylase activity showed normal cardiovascular function and normal /3-hydroxylation of an administered synthetic substrate, hydroxyamphetamine. Pronounced blood pressure reduction by antihypertensive drugs did not change serum dopamine-/J-hydroxylase activity, and intensive bicycle exercise produced only small and variable increases. Two subjects with severe sympathetic dysfunction, i.e., orthostatic hypotension, showed normal levels of dopamine-/?-hydroxylase activity (89-123 units/ml). Serum dopamine-)6-hydroxylase activity varied from 450 to 950 units/ml in five subjects with pheochromocytomas and decreased to one-tenth the preoperative level in one subject who underwent curative surgery. Therefore, serum dopamine-/3-hydroxylase activity is not a satisfactory index of sympathetic function; moreover, activity levels do not correlate with prevailing blood pressure. Subjects with pheochromocytomas, however, may show increased serum levels of the enzyme, which demonstrates that such tumors can release dopamine-/3-hydroxylase.