Krebs G
Biochimie. 1979;61(4):559-65. doi: 10.1016/s0300-9084(79)80212-6.
The complex formed between trypsin (Tn) and alpha 2 Macroglobulin (alpha 2 M) retains the whole hydrolytic activity of the enzyme for synthetic substrates. Moreover synthetic inhibitors of low molecular weight stiel inhibit this activity. A comparative study of three inhibitors (Benzylamine, Butylamine, Benzamidine) has been carried out and shows that their behavior is similar. These inhibitors bind trypsin when it is bound to alpha 2 M and reciprocally alpha 2 M can bind Tn-inhibitor complex. Nevertheless the dissociation constant of the enzyme-inhibitor complex (Ki) is increased by alpha 2 M. In the case of Benzamidine the value of Ki is 2.22.10(-5) M for native enzyme and 13.4.10(-5) M for Tn-alpha 2 M and in the case of Butylamine this value increases from 0.5.10(-3) M to 2.95.10(-3) M. These variations of the Ki values are due to the modification of the accessibility of the inhibitor to the active site. Unpublished results show that the alpha 2 M molecule undergoes a deep structural modification in the course of the complex formation, which must lead to an increase of the value of Ki. This structural modification is probably irreversible so that the alpha 2 M complex has never been dissociated without altering the alpha 2 M molecule. The increase of the values of Ki cannot therefore result in an effective decrease of the association constant of the Tn-alpha 2 M complex.
胰蛋白酶（Tn）与α2巨球蛋白（α2M）形成的复合物保留了该酶对合成底物的全部水解活性。此外，低分子量的斯蒂尔合成抑制剂可抑制这种活性。对三种抑制剂（苄胺、丁胺、苯甲脒）进行了比较研究，结果表明它们的行为相似。当胰蛋白酶与α2M结合时，这些抑制剂会与胰蛋白酶结合，反之，α2M也能与Tn-抑制剂复合物结合。然而，α2M会增加酶-抑制剂复合物（Ki）的解离常数。就苯甲脒而言，天然酶的Ki值为2.22×10⁻⁵M，Tn-α2M的Ki值为13.4×10⁻⁵M；就丁胺而言，该值从0.5×10⁻³M增加到2.95×10⁻³M。Ki值的这些变化是由于抑制剂对活性位点的可及性发生了改变。未发表的结果表明，α2M分子在复合物形成过程中经历了深度的结构修饰，这必然导致Ki值增加。这种结构修饰可能是不可逆的，因此，在不改变α2M分子的情况下，α2M复合物从未被解离过。因此，Ki值的增加不会导致Tn-α2M复合物结合常数的有效降低。
