Basic nutritional investigationIsoflavonoids and peptides from meju, long-term fermented soybeans, increase insulin sensitivity and exert insulinotropic effects in vitro
Introduction
Type 2 diabetes mellitus emerges from uncompensated peripheral insulin resistance that is associated with unregulated nutrient homeostasis, obesity, and peripheral insulin resistance and progressive β-cell failure [1]. Recent studies on experimental animals have shown that the failure of insulin secretion causes the development of type 2 diabetes, which is associated with decreased β-cell expansion [2], [3]. Thus, the development or progression of type 2 diabetes can be prevented or delayed by promoting glucose-stimulated insulin secretion and by intervening in the onset of insulin resistance [4].
The most promising classes of agents that target antidiabetic action are peroxisome proliferator-activated receptor-γ (PPAR-γ) agonists and glucagon-like peptide-1 (GLP-1) receptor agonists. Although the precise mechanisms of the action of PPAR-γ agonists such as troglitazone, rosiglitazone, and pioglitazone have not been fully revealed, the end results of their physiologic effects have been to decrease insulin resistance in adipose tissues, muscles, and the liver [5], [6]. Another target for alleviating diabetes is the GLP-1 receptors. GLP-1 and their long-acting agonists, such as exendin-4, improve β-cell functions such as glucose-stimulated insulin secretion and β-cell mass as shown by in vitro and in vivo studies [7], [8]. These agonists can be found naturally in foods.
Soybeans (Glycine max MERILL) have long been consumed as an important protein source to complement grain proteins in Asian countries. In addition to protein, they contain various nutritious and functional components such as isoflavonoids, which are helpful in protecting against metabolic diseases [9]. Various food products have been derived from soybeans in Asian countries. Fermentation is one of the major processes associated with the production of food from soybeans. In Korea, there are several traditional fermented soybean products, the most commonly used being chungkookjang, doenjang, kochujang, and soy sauce. Doenjang, kochujang, and soy sauce are fermented with different micro-organisms when traditionally made, because fermentation conditions and micro-organisms in the environment vary (Fig. 1). To make these products by the traditional method, cooked soybeans are formed into blocks and fermented outdoors for 20–60 d by micro-organisms naturally present in the environment. The fermented blocks are known as meju and are used to prepare the soy pastes and sauce. Soybeans are typically fermented primarily by Bacilli species during the early stages of fermentation, followed by Aspergillus species, which are predominant after several days. Aspergillus oryzae is the major micro-organism in the final product of meju when it is made in the traditional way. Little research has been devoted to meju compared with doenjang, kochujang, and soy sauce, probably because it is not the final product to be consumed. Doenjang, kochujang, and soy sauce are made from meju and other ingredients such as salts and red pepper. The final soy products are very high in salt; however, meju has undergone most of the fermentative transformations and contain many novel compounds. Therefore, it is important to study meju to determine the functional properties of this long-term fermented soy product that contains no salt. Meju has the potential to be developed as a functional food or as a source of novel bioactive compounds. In the present study, we compared the insulin-sensitizing and insulinotropic actions of unfermented and long-term fermented soybeans (meju fermented for 20 or 60 d) and individual isoflavonoid aglycones in 3T3-L1 adipocytes, NCI-H716 cells (GLP-1–producing cells), and Min6 cells (mouse insulinoma cells). In addition, the mechanisms responsible for these effects were investigated.
Section snippets
Preparation and extraction of meju
Meju was generated by a traditional processing method at a manufacturing company in the Sunchang Fermented Soy Product Valley (Sunchang, Korea). Soybeans were sorted, washed, and soaked in water for 18–20 h at 20 °C and steamed for 30 min at 121 °C. The steamed soybeans were cooled to 40 °C and molded into blocks, which were 10 × 8 × 20 cm, and hung up outdoors for 2 mo. Samples were collected at 0, 20, and 60 d during the fermentation process and each sample of meju collected on these days was
Major compounds in methanol and water extracts of meju
During the fermentation process isoflavonoids and protein profiles were altered (data not shown). The methanol extracts from meju mostly contained isoflavonoids, whereas the water extracts had proteins and peptides. Protein separation by tricine-SDS-PAGE revealed that larger peptides were degraded into smaller ones as the fermentation periods increased (Table 1). Most proteins larger than 20 kDa were degraded into smaller proteins in the meju water extracts that had been fermented longer than
Discussion
Soybeans contain 12 isoflavonoids, which appear to be genistein, glycitein, daidzein and their derivatives, with glucose, malonyl, and acetyl groups attached. Fermentation alters the profiles of isoflavonoids and peptides. A previous study by our research team showed that. as the fermentation periods became longer, the quantities of malonyl, acetyl, and glycosidyl isoflavonoids decreased during fermentation but those of isoflavonoid aglycones, daidzein, and genistein were increased by over 10-
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This work was supported by a grant from the Korea Science and Engineering Foundation in Korea (M10510120001-05N1012-00111).