Improved hydrolysis of α-tocopherol acetate emulsion and its bioaccessibility in the presence of polysaccharides and PEG2000

Abstract

Dietary macromolecules are often used for food formulation. Most of the published literature usually focuses on a certain polysaccharide reducing lipid digestion, but the non-specific physical effects such as the excluded volume effect of the surrounding macromolecules, which will possibly enhance the hydrolysis of lipids, is rarely discussed. As the influence of polysaccharides on the gastrointestinal fate of α-tocopherol acetate (VE-A) encapsulated in emulsion have not been conducted so far, in this research, we study the effect of crowded medium created by polysaccharides (Ficoll400) on the hydrolysis of VE-A and its bioaccessibility. The results show that the presence of Ficoll400 increased the bioaccessibility and conversion of VE-A 1.6 times that of a dilute buffer. An enzyme-catalyzed kinetic reaction rate of VE-A hydrolysis is increased up to 3.5 times. Both the improved thermodynamic conversion yields and kinetic rates of the hydrolysis of the VE-A emulsion was further observed in a synthetic macromolecule PEG2000 medium. This research improves our understanding of the crowding effect of macromolecules on VE-A/lipid emulsion digestion.

Introduction

Vitamin E (VE) is found naturally in a wide variety of foods, and is available as a dietary supplement due to its strong antioxidant activity and potential health benefits [1]. A more chemically stable derivative, α-tocopherol acetate (VE-A), is typically used in food, pharmaceuticals, and cosmetic products [2]. Among the various colloidal delivery systems that have been developed, oil-in-water emulsions are the most commercially available for VE-A encapsulation [3]. During digestion, fat-soluble VE-A is released from the lipid droplets and incorporated into the hydrophobic interior of the microaggregates. Before reaching the epithelium cells where they may be absorbed, VE-A undergoes solubilization in mixed micelles (bioaccessibility) and chemical hydrolysis (conversion) [4]. Studies have shown that the bioaccessibility of VE emulsions is affected by the chain length of the fatty acid of the triglyceride (TG) used as carrier oil [5,6]. Another study shows that the micelle solubilization of VE could be increased by the presence of phospholipids, but does not depend strongly on the presence of free fatty acid (octanoic acid or linoleic acid) [7]. Consequently, the bioavailability of VE would be increased if there were greater conversion of VE-A to VE in the gastrointestinal tract. These results suggest that the bioaccessibility of VE encapsulated in emulsion-based delivery systems and the hydrolysis of VE-A to VE is strongly influenced by carrier oil type, surfactant concentration, calcium ions, and phospholipids [8].

Polysaccharides are often used to increase emulsion stability by adsorbing at the droplet surface, thus decreasing the interfacial tension and enhancing the interfacial elasticity or increasing the viscosity of the continuous phase [9]. The polysaccharides adsorption layer can provide electrostatic and steric stabilization, thus improving emulsion thermal stability and resistance to external treatment. Gudipati et al. [10] used chitosan and sodium alginate to prepare a multilayer emulsion, and found that polysaccharide coating reduced lipid digestion. The formation of calcium alginate gel limited the acquisition of lipid by pancreatin [11]. Pectin or chitosan promoted corn oil droplet aggregation, thereby reducing the contact area between oil and digestive enzymes and, thus, reducing lipid digestibility [12]. Though the effect of polysaccharides on lipid digestion attracted researcher’s interest, studies on the influence of polysaccharides in the gastrointestinal fate of VE-A encapsulated in emulsion have not been conducted. Moreover, most researchers were concerned about the emulsions stabilized by biopolymer and the reduced lipid digestibility by polysaccharides. However, the impact of the excluded volume effect created by macromolecules, which will possibly enhance the lipid hydrolysis, has been neglected.

“Macromolecular crowding” refers to nonspecific interactions between macromolecules and the surroundings within a crowded medium that likely influence the equilibrium and rates of reactions in which macromolecules participate. The excluded volume effect of the crowded surroundings changes the thermodynamic activities of dissolved molecules and, consequently, affects the chemical equilibria and reaction rates [13]. In the digestion process, it is inevitable that macromolecules, such as polysaccharides, proteins, and enzymes create a crowded medium, which would affect the interaction of lipase with VEA loaded oil droplets. Most of the published work on the crowding effect focuses on the influence of mutual volume exclusion on the energetics and transport properties of macromolecules such as proteins or nucleic acids [14,15]. We previously found that macromolecular crowding affects hydrolysis reaction on vesicles interfaces that involved fatty acids [16,17]. Binding of long- and medium-chain fatty acids with serum albumin in macromolecular crowding was also completely different than that in dilute buffer [18]. Because pancreatic lipases are interfacial enzymes that can be activated at oil–water interfaces to access water insoluble substrates for hydrolysis, we hypothesize that the excluded volume effect of the surrounding macromolecules in the small intestine may facilitate the interaction of the enzyme and the substrates, and therefore promote the hydrolysis of lipids. Recent work in our group reveals that the kinetic parameters of pancreatin in crowded surroundings can be modulated by a dietary polysaccharide and are dependent on the fatty acid chain length of triacylglycerols (TGs) [19]. The question thus arises: if the TGs act as carrier oils, can the digestion of fat-soluble vitamins inside the oil droplets be promoted by a crowded medium?

In this research, the influence of polysaccharides Ficoll400 and synthetic macromolecules PEG2000 on VE-A emulsions were investigated, including the thermodynamic conversion efficiency (bioaccessibility) measured by HPLC, and the kinetic parameters of VE-A hydrolysis measured by isothermal titration calorimetry (ITC). The impact of the carrier oil chain length was also investigated. The results are useful for understanding the physicochemical rules of the macromolecular crowding effect on digestive enzyme behavior and VE-A transformation in a more simulated physiological environment.