Selenium accumulation in protein fractions of Tenebrio molitor larvae and the antioxidant and immunoregulatory activity of protein hydrolysates
Introduction
Selenium is an essential micronutrient that is required to maintain the health of humans and animals (Kieliszek, 2019). Selenium deficiency is responsible for the development of various metabolic syndromes, chronic diseases, and even cancers (Kieliszek & Blazejak, 2013). However, the levels of selenium in daily food such as cereals, grains, fruits, vegetables, and meats, are relatively low. Therefore, dietary supplementation with selenium is necessary for the maintenance of human health. Selenium exists in inorganic and organic forms. Organic selenium is safer than inorganic selenium and exhibits higher bioavailability. Many researches have used edible microorganisms, plants and livestock to produce organic selenium. However, some selenium source exhibit shortcomings that include a low content of organic selenium, a long production period, and a high cost (Zhang et al., 2020). Therefore, a new selenium-enriched method allowing high organic selenium yield in a short production period and at a low cost is still required.
Insects are a type of invertebrates, which have been used as a source of food for centuries (Yi, Lakemond, Sagis, Eisner-Schadler, van Huis, & van Boekel, 2013). Tenebrio molitor (TM) commonly known as the yellow mealworm (Coleoptera: Tenebrionidae), has increasingly been in demand as a promising source of protein for animal feed and food products (Siemianowska et al., 2013). Organic selenium is often found in proteins in the form of selenoamino acid, such as selenomethionine (SeMet), selenocysteine (SeCys). Foods that can be used as a source of dietary supplementation of selenium are typically rich in protein, and these foods include selenium-enriched soybean, selenium-enriched egg, and selenium-enriched milk. TM larvae (TML) are rich in proteins (24.3%–27.6% in wet matter), and the enzymatic hydrolysates of these proteins possess numerous biological properties, such as antioxidant and immunoregulatory activities, that are beneficial to human health (Nongonierma & FitzGerald, 2017). Additionally, TML grows rapidly, and breeding of this organism can be performed at a low-cost and is less burdensome to the environment (Siemianowska et al., 2013). Importantly, TML exhibit a strong ability to utilize inorganic waste (Bednarska and Swiatek, 2016, Yang et al., 2019). Therefore, this edible insect possesses potential as a new candidate for selenium enrichment.
Previous studies have demonstrated that biological selenium accumulation can affect the metabolism of sugars, fats, proteins, and other physiological process within organisms (Weekley & Harris, 2013). This indicated that selenium enrichment can not only result in an increase of total selenium content but can also affect the content and composition of other nutrients, such as amino acids. Certain active peptides derived from food sources are produced by enzymatic hydrolysis of proteins that are often used as a type of functional food additive. It has been previously reported that selenium enrichment also affects the biological activity of enzymatic hydrolysates (Ye, Wu, Zhang, & Wang, 2019).
In the present study, a means of achieving selenium enrichment through the use of TML was developed. The distribution of selenium within the protein was investigated. Additionally, the effect of selenium accumulation on the protein composition and amino acids of TML and the antioxidant and immunoregulatory activities of the intrinsic protein hydrolysate were also evaluated. The results of this study may provide a novel and promising means of selenium enrichment.
Section snippets
Materials
The second instar yellow mealworm larvae each weighing 40–45 mg were obtained from a commercial supplier (SJN biotechnology Co, Guangzhou, China). Alcalase (200 μ/mg), trypsin (250 μ/mg), neutrase (60 μ/mg), and flavourzyme (90 μ/mg) were purchased from Novozymes (Bagsvaerd, Denmark). H2O2, 1-diphenyl-2-picrylhydrazyl (DPPH), AAPH, 2′,7′-dichlorofluorescein diacetate (DCFH-DA), Dulbecco’s modified eagle’s medium (DMEM), fetal bovine serum (FBS), lipopolysacch aride (LPS), penicillin, and
Accumulation of selenium in TML
As shown in Table 1, the presence of sodium selenite at concentrations greater than 20.0 μg/g in the feed significantly suppressed the survival rate of TML and their ability to accumulate inorganic selenium. According to previous data, the safe dose of sodium selenite in the basal diet for pigs and chickens is approximately 1 μg/g, and this is the reason that the use of livestock to produce selenium-enriched foods often requires the addition of organic selenium to their feed (Zhang, et al., 2020
Conclusions
The present study demonstrated that TML could accumulate and transform inorganic selenium into its organic form. The total content of selenium in the larvae was 54.21 ± 1.25 μg/g after administering the larvae with 20 μg/g of sodium selenite through the feed, and of this content, the organic form accounted for over 97%. Approximately 81% of the selenium found in soluble proteins was distributed in subunits with a Mw of <40 kDa. After hydrolysis using an alkaline protease, the hydrolysate of
Author contributions
Wu hui: ideas.
Dong zhou: development of methodology, preparation of the published work.
Zhang mengmeng: creation of models.
Lin yanyin: specifically performing the experiments.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the China Postdoctoral Science Foundation (Nos. 2019M662931).
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