Inhibition of dopamine beta-hydroxylase by bidentate chelating agents.
1-2H-Phthalazine hydrazone (hydralazine; HYD), 2-1H-pyridinone hydrazone (2-hydrazinopyridine; HP), 2-quinoline-carboxylic acid (QCA), 1-isoquinolinecarboxylic acid (IQCA), 2,2'-bi-1H-imidazole (2,2'-biimidazole; BI), and 1H-imidazole-4-acetic acid (imidazole-4-acetic acid; IAA) directly and reversibly inhibit homogeneous soluble bovine dopamine beta-hydroxylase (3,4-dihydroxyphenethylamine, ascorbate:oxygen oxidoreductase (beta-hydroxylating), EC 1.14.17.1). HYD, QCA and IAA show competitive allosteric inhibition of dopamine beta-hydroxylase with respect to ascorbate (Kis = 5.7(+/- 0.9) microM, 0.14(+/- 0.03) mM, 0.80(+/- 0.20) mM; nH = 1.4(+/- 0.1), 1.8(+/- 0.4), 2.8(+/- 0.6), respectively). HYD and IAA show slope and intercept mixed-type allosteric inhibition of dopamine beta-hydroxylase with respect to tyramine. QCA shows allosteric uncompetitive inhibition of dopamine beta-hydroxylase with respect to tyramine. HP, BI and IQCA all show linear competitive inhibition (Kis = 1.9(+/- 0.3) microM, 21(+/- 6) microM, and 0.9(+/- 0.3) microM, respectively) with respect to ascorbate. HP and BI show linear mixed-type while IQCA shows linear uncompetitive inhibition of dopamine beta-hydroxylase with respect to tyramine. In the presence of HP, HYD or IAA intersecting double-reciprocal plots of the initial velocity as a function of tyramine concentration at differing fixed levels of ascorbate are observed. These findings are consistent with a uni-uni-ping-pong-ter-bi kinetic mechanism for dopamine beta-hydroxylase that involves a ternary enzyme-ascorbate-tyramine-oxygen complex. The results for HYD, QCA and IAA are the first examples of allosteric inhibitor interactions with dopamine beta-hydroxylase.
Townes
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Department of Chemistry, Howard University, Washington, DC 20059.
Titone
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Rosenberg
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D000818:Animals; D001667:Binding, Competitive; D002614:Chelating Agents / Q000494:pharmacology*; D004299:Dopamine beta-Hydroxylase / Q000037:antagonists & inhibitors* / Q000302:isolation & purification; D006632:Histamine / Q000031:analogs & derivatives / Q000494:pharmacology; D006830:Hydralazine / Q000494:pharmacology; D007093:Imidazoles; D007700:Kinetics; D008433:Mathematics; D011804:Quinolines / Q000494:pharmacology
D002614:Chelating Agents; D007093:Imidazoles; D011804:Quinolines; D006830:Hydralazine; D006632:Histamine; D004299:Dopamine beta-Hydroxylase; C005954:imidazoleacetic acid; C008898:quinaldic acid