Townes S, Titone C, Rosenberg R C
Department of Chemistry, Howard University, Washington, DC 20059.
Biochim Biophys Acta. 1990 Feb 9;1037(2):240-7. doi: 10.1016/0167-4838(90)90174-e.
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.
1-2H-酞嗪腙（肼屈嗪；HYD）、2-1H-吡啶酮腙（2-肼基吡啶；HP）、2-喹啉羧酸（QCA）、1-异喹啉羧酸（IQCA）、2,2'-联-1H-咪唑（2,2'-联咪唑；BI）和1H-咪唑-4-乙酸（咪唑-4-乙酸；IAA）可直接且可逆地抑制纯可溶性牛多巴胺β-羟化酶（3,4-二羟基苯乙胺，抗坏血酸：氧氧化还原酶（β-羟化），EC 1.14.17.1）。HYD、QCA和IAA对多巴胺β-羟化酶表现出相对于抗坏血酸的竞争性变构抑制作用（抑制常数分别为5.7（±0.9）微摩尔/升、0.14（±0.03）毫摩尔/升、0.80（±0.20）毫摩尔/升；希尔系数分别为1.4（±0.1）、1.8（±0.4）、2.8（±0.6））。HYD和IAA对多巴胺β-羟化酶表现出相对于酪胺的斜率和截距混合型变构抑制作用。QCA对多巴胺β-羟化酶表现出相对于酪胺的变构非竞争性抑制作用。HP、BI和IQCA对抗坏血酸均表现出线性竞争性抑制作用（抑制常数分别为1.9（±0.3）微摩尔/升、21（±6）微摩尔/升和0.9（±0.3）微摩尔/升）。HP和BI表现出线性混合型抑制作用，而IQCA对多巴胺β-羟化酶表现出相对于酪胺的线性非竞争性抑制作用。在存在HP、HYD或IAA的情况下，观察到在不同固定抗坏血酸水平下，初始速度作为酪胺浓度函数的相交双倒数图。这些发现与多巴胺β-羟化酶的单-单-乒乓-双动力学机制一致，该机制涉及三元酶-抗坏血酸-酪胺-氧复合物。HYD、QCA和IAA的结果是变构抑制剂与多巴胺β-羟化酶相互作用的首个实例。
