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

多项证据表明，大豆蛋白的某些成分参与了脂质代谢的调节，其中一种参与代谢的激素是胰岛素。我们寻找能够影响胰岛素作用的大豆蛋白成分。通过对纯化肽的测序和对离体大鼠脂肪细胞脂肪分解的影响，证实了甘氨酸的A_<1a>和A_2酸性亚基在体外增强胰岛素的作用。此外，这些蛋白质显示出与胆汁酸结合的能力。因此，这些蛋白可能是通过干扰类固醇吸附来发挥降低胆固醇作用的候选蛋白，这是大豆蛋白的一种众所周知的生理功能。然而，胆汁酸结合能力与胰岛素刺激活性无关。这两种特性在豌豆种子中豆科蛋白的酸性亚基中都有发现，表明这些特性可能是豆科蛋白所共有的。A_<1a>亚基胰蛋白酶化产物（A_<1a>/TR）也表现出胰岛素刺激活性，并利用大鼠脂肪细胞研究了其分子机制。A_<1a>/TR不影响葡萄糖转运，而葡萄糖转运是胰岛素作用于细胞的主要过程。然而，A_<1a>/TR增强了脂肪分解对胰岛素的反应性和敏感性。胰岛素对细胞表面受体的亲和力不受A_<1a>/TR的影响。A_<1a>/TR保护胰岛素不降解，导致细胞内胰岛素浓度升高。在15ºC时，没有观察到胰岛素刺激活性，这表明胰岛素的内在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对激素的最大反应中观察到的增强作用。

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.