O'Neal S G, Earle S R, Fisher R R
Biochim Biophys Acta. 1980 Feb 8;589(2):217-30. doi: 10.1016/0005-2728(80)90039-0.
Bovine heart submitochondrial particle transhydrogenase is inhibited by cations in a concentration and pH-dependent manner, and non-energy-linked transhydrogenation is inhibited to a greater extent by metals than the energy-linked reaction. The inhibition of the enzyme by Mg2+ is competitive with the NADP substrate and non-competitive with the NAD substrate. Mg2+ stimulates inactivation of the enzyme by 5,5'-dithiobis(2-nitrobenzoic acid), and protects against thermal and proteolytic inactivation. This suggests that Mg2+ binding in the NADP site alters transhydrogenase to a more thermostable conformation, which is less susceptible to attack by trypsin and more reactive with 5,5'-dithiobis(2-nitrobenzoic acid). Other cation inhibitors mimic Mg2+ in these properties. The order of effectiveness of the inhibitors tested is La3+ greater than Mn2+ greater than Ca2+ congruent to Mg2+ greater than Sr2+ greater than Na+ congruent to K+. This order is described by the Irving-Williams order for the stability of metal-ligand complexes, suggesting that carboxylates or amines may comprise the inhibitory cation binding site.
牛心亚线粒体颗粒转氢酶受到阳离子的抑制，其抑制作用呈现浓度和pH依赖性，并且与能量偶联反应相比，非能量偶联的转氢作用受到金属的抑制程度更大。Mg2+对该酶的抑制作用与NADP底物存在竞争性，而与NAD底物不存在竞争性。Mg2+可促进5,5'-二硫代双(2-硝基苯甲酸)对该酶的失活作用，并能防止其热失活和蛋白水解失活。这表明在NADP位点结合的Mg2+会使转氢酶转变为更耐热的构象，这种构象更不易受到胰蛋白酶的攻击，且与5,5'-二硫代双(2-硝基苯甲酸)的反应性更高。其他阳离子抑制剂在这些特性上可模拟Mg2+。所测试抑制剂的有效性顺序为：La3+＞Mn2+＞Ca2+≈Mg2+＞Sr2+＞Na+≈K+。该顺序与金属-配体配合物稳定性的欧文-威廉姆斯顺序相符，这表明羧酸盐或胺可能构成了抑制性阳离子结合位点。
