Production of low-molecular weight soluble yeast β-glucan by an acid degradation method

Yeast have long been applied to produce many fermented goods such as bread, beer, and wine. The biological activity of yeast themselves has been precisely and extensively analyzed, and many new products have been developed as functional foods. The yeast cell wall is composed of mannoproteins in the outer layer and β-1,3-/β-1,6-glucan and chitin in the inner layer. These macromolecules are linked to each other. β-Glucan performs an important function in the architecture of the yeast cell wall. Fungal β-glucan is synthesized as a soluble polymer and is incorporated into nascent cell walls to build an essentially water-insoluble macromolecule. Therefore, the yeast cell wall is insoluble and difficult to dissolve in water.

β-Glucan is widespread in nature and has many useful biological activities. For example, it exerts antitumor effects acting via macrophages, natural killer cells, and killer T cells because of activation of the host immune system [1], as well as anti-inflammatory effects via immunomodulation of cells involved in innate immune response [2], [3], and suppression of cholesterol absorption as a water-soluble dietary fiber [4], [5]. These reports mention the utility of many soluble β-glucans, and solubilizing insoluble yeast β-glucan may further expand the possible applications of β-glucans.

β-Glucans are recognized by dectin-1 and CR3 receptors [6], activating both innate and acquired immunity. In particular, dectin-1 has been heavily studied as a receptor for β-glucan [7], [8], [9]. It is expressed in innate immune cells in mice, such as macrophages, dendritic cells, and neutrophils. β-Glucan binds to dectin-1 regardless of its solubility, but its activity is known to change with solubility. Previously, we prepared soluble and insoluble β-glucans from Candida albicans and analyzed their biological activities [10]. We previously reported that insoluble β-glucan strongly affects TNF-α and reactive oxygen species (ROS) production. In contrast, soluble β-glucan has a different effect: it activates innate as well as acquired immunity, e.g., T cells. In addition, ROS production in response to the insoluble β-glucan is suppressed by soluble β-glucan. Significant differences are observed in gene expression when peripheral blood mononuclear cells (PBMCs) are stimulated with soluble β-glucan versus insoluble β-glucan [11]. Furthermore, the formation of a cluster of dectin-1 is important for dectin-1 signaling, and only insoluble β-glucan, which aggregates many dectin-1 molecules, transmits a signal. On the other hand, soluble β-glucan has been reported to exert immune activity via dectin-1 stimulation [12]. That is, the structure and properties differ depending on the origin of β-glucans, as does the biological activity.

Generally, insoluble β-glucan from the yeast cell wall can be utilized as food. Soluble yeast β-glucan may have many uses, but the cell wall β-glucan is robust and difficult to solubilize. Laminarin derived from seaweed is widely used as a soluble small molecule β-glucan. However, the quality of seaweed changes greatly depending on harvest location and season. Therefore, we focused on β-glucan from yeast because it can be mass cultured and can maintain consistent quality. Previously, we reported a method to solubilize yeast cell wall β-glucan using heat degradation, and succeeded in preparing large soluble polysaccharide fractions instead of low-molecular weight oligosaccharides [13]. Heat-degraded soluble β-glucan (hd-sBBG) showed antagonistic activity and reacted strongly with dectin-1, an innate immunity receptor of β-glucan. Therefore, we developed a method to prepare low-molecular weight solubilized yeast cell wall β-glucan using sulfuric acid, which can be used as a food additive. In this study, we investigated physical properties and immunomodulatory effects of low-molecular weight soluble β-glucan degraded by a novel acid decomposition method.