Serum dopamine-beta-hydroxylase activity.

Dopamine-yS-liydroxylase, the enzyme which converts dopamine to norepinephrine, is released into the perfusate upon stimulation of the isolated perfused adrenal gland and after stimulation of the nerves to the isolated perfused spleen. This study was undertaken to determine whether dopamine-/?hydroxylase activity could be detected circulating in blood. By using a sensitive new enzymatic assay, a dopamine-/3-hydroxylase activity was found in the blood of both man and the rat. It is located in the serum and is not associated with the formed elements of blood. The serum activity is similar to that of purified bovine adrenal dopamine-^3-hydroxylase in that it requires the presence of ascorbic acid, catalase, fumarate and oxygen for full activity. Furthermore, as is also the case with the adrenal enzyme, serum activity is increased in the presence of cupric ions. The Km values for substrate in human and rat sera are similar, and both are close to values determined in rat adrenal glands and stellate ganglia. The mean activity ± S E in the serum of six rats was 2.27 ± .04 nmoles/ml serum/ 20 min, and that of four normal humans ranged from 96.2 to 284 nmoles/ml/20 min.