Insulin-modulating peptide from soybean protein: Identification of the component and the mechanism on biological activity

一些证据表明，大豆蛋白的某些成分参与脂质代谢的调节，并且参与代谢的激素之一是胰岛素。我们寻找能够影响胰岛素作用的大豆蛋白组分。通过对纯化肽的序列测定和对离体大鼠脂肪细胞脂肪分解作用的研究，证实<1a>大豆球蛋白的A_1和A_2酸性亚基在体外可增强胰岛素的作用。此外，蛋白质显示出结合胆汁酸的能力。因此，这些蛋白质是通过干扰类固醇吸附而发挥降低胆固醇作用（大豆蛋白的一种众所周知的生理功能）的可能候选物。然而，胆汁酸结合能力与胰岛素刺激活性无关。胰蛋白酶消化的A_<1a>亚基产物（A_<1a>/TR）也具有胰岛素刺激活性，并利用大鼠脂肪细胞研究了其分子机制。A_<1a>/TR不影响胰岛素作用于细胞的主要过程--葡萄糖转运，但<1a>能增强脂肪分解对胰岛素的反应性和敏感性，不影响胰岛素与细胞表面受体的亲和力<1a>。A_<1a>/TR保护胰岛素不被降解，导致细胞内胰岛素浓度增加。在15 ℃下未观察到胰岛素刺激活性，表明胰岛素的内化是蛋白质效应出现的必要条件，单独抑制胰岛素降解不能解释在对激素的最大反应中观察到的增强效应。

Several lines of evidence had indicated that some component(s) of soybean protein participates in the modulation of lipid metabolism and that one of the hormones involved in the metabolism is insulin. We looked for soybean protein component(s) which is capable of affecting the insulin action. By sequencing of purified peptides and examination of their effects on fat decomposition in isolated rat adipocytes, it was confirmed that A_<1a> and A_2 acidic subunits of glycinin potentiate the action of insulin in vitro. In addition, the proteins showed an ability to bind bile acids. Thus, these proteins are probable candidates which exert the hypocholesterolemic effect, a well-known physiological function of soybean protein, by interfering with steroid adsorption. However, the bile acidbinding ability was not related to the insulin-stimulating activity. Both properties were noted in acidic subunits of legumin from pea seed, indicating that these characteristics may be common to legume proteins.Trypsinized products of A_<1a> subunit (A_<1a>/TR) also exhibited the insulin-stimulating activity and the molecular mechanism was examined by using rat adipocytes. A_<1a>/TR did not affect the glucose transport, which is the primary process of insulin action on the cells.However, A_<1a>/TR enhanced a reactivity and a sensitivity of fat decomposition to insulin.Affinity of insulin to its receptor on the cell surface was not affected by A_<1a>/TR. A_<1a>/TR protected insulin from degradation, resulted in an increase of insulin concentration in the cells. The insulin-stimulating activities were not observed at 15ﾟC, indicating that the internarization of insulin is requisite for an appearance of the protein effect, and the inhibition of insulin degradation alone cannot account for the enhancing effect observed in the maximal responses to the hormone.