Dopamine-(9-Hydroxylase is an enzyme which catalyzes dopamine to noradrenaline in final step of catecohlamine synthesis. This study was undertaken to determine the human serum Dopaminelj3-Hydroxylase activity and examine effects of fusaric acid (5-butylpicolinic acid) on this enzyme activity in vitro and in vivo. A sensitive method for measurement of the human serum Dopamine-8-Hydroxylase activity was established and it was important for the assay to adjust PH of reaction mixtures to PH 4.5.

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... tures to PH 4.5. The activity ranged from 39.95 to 176.00, mean was 82.31+33.11 (M. +S.E.) in normal and that of patients with essential hypertension ranged from 31.11 to 234.97, mean was 109.87-1-57.23 (M.+S.E.) n moles/0.5 ml serum/15 min. Fusaric acid showed 50% inhibition of the human serum Dopamine-fi-Hydroxylase activity at concentration of 10-7 M in vitro and the activity in patients with essential hypertension was significantly decreased in a few days after the administration of fusaric acid 150 mg per day. (See pp. 48,49) Vol. 49 No. 1