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1.Two different molecular forms of dopamine-ß-hydroxylase were isolated from human serum; a major component (Peak I enzyme) with a molecular weight of 368000 and with a higher specific activity and a minor component (Peak II enzyme) with a molecular weight of 188000 and with a lower specific activity.2.Both forms require ascorbic acid for the activity, and are stimulated by fumarate. Addition of n-ethylmaleimide or copper also increased the activity. The optimal pH of both forms in the presence of 20mm tyramine as substrate is 5.0.3.Km values toward tyramine of Peak I enzyme and Peak II enzyme were 1.67mm and 14.2mm, respectively.4.Both Peak I enzyme and Peak II enzyme are glycoprotein.