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—Tyramine β-hydroxylase catalyzes the biosynthesis of octopamine in the lobster nervous system. This enzyme has been characterized and a rapid microassay, based on the enzymic release of tritiated water from [1,2-(side chain) 3H] tyramine, has been developed. Lobster tyramine β-hydroxylase resembled mammalian dopamine β-hydroxylase. The most conspicuous differences were that the lobster enzyme was inhibited by anions, particularly fumarate, and had a higher affinity for substrates. Tyramine β-hydroxylase activity was present in both particulate and soluble fractions of homogenates of the lobster nervous system. Bound activity, extracted by repeated freezing and thawing, was partially purified. The enzyme had the following properties: (1) The optimum pH for the conversion of tyramine to octopamine was 7·4. (2) The apparent Michaelis constant for tyramine was 0·15 mm and for ascorbic acid was 0·2 mm at pH 6·6. (3) The purified enzyme was inhibited by salts; the degree of inhibition was sensitive to the anion and decreased in the order chloride ⋍ fumarate > sulphate > acetate. (4) Tyramine β-hydroxylase was inhibited by metal chelating agents and by cupric sulphate at concentrations greater than 10−4m; N-ethylmaleimide had no significant effect on activity in concentrations up to 3 mm. (5) The purified enzyme also β-hydroxylated dopamine to form norepinephrine, with an apparent Michaelis constant of 0·24 mm. This activity co-purified with tyramine β-hydroxylase, suggesting that a single enzyme catalyzed both reactions. |
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