Effect of atmospheric cold plasma on structure, activity, and reversible assembly of the phytoferritin
Introduction
Atmospheric cold plasma (ACP), a non-thermal technology, refers to the non-equilibrium plasma generated at near-ambient temperature and pressure (Han et al., 2016, Terpiłowski et al., 2017). It is a source of reactive oxygen species, including singlet oxygen and ozone, and can excite molecular nitrogen (Misra, Pankaj, Segat, & Ishikawa, 2016). ACP has been widely applied in food preservation by taking advantage of its inactivation effect on microorganisms, including spoilage organisms and food borne pathogens (Cheng et al., 2014, Han et al., 2014, Misra et al., 2016). ACP has shown potential for application in surface hydrophobicity enhancement (Misra et al., 2014), surface modulation (Bahrami et al., 2013), and enzyme inactivation (Mastwijk & Groot, 2010), in addition to food preservation. The applications of ACP have been expanded to include the treatment of biological macromolecules, such as whey protein (Segat, Misra, Cullen, & Innocente, 2015). However, studies are rarely reported on the effect of ACP treatment on the structure and property of ferritin, an iron storage protein that is widely distributed in plants, animals, and bacteria.
Ferritin is a cage-like protein that can store thousands of iron ions in a nanosized cavity (Yang, Zhou, Sun, Gao, & Xu, 2015). Each ferritin consists of 24 similar or different protein subunits. These subunits assemble into a shell-like structure with an internal diameter of 8 nm and an external diameter of 12 nm. Ferritin is principally characterized by iron oxidative deposition and iron release via the 3-fold or 4-fold channels (Chasteen & Harrison, 1999). Another distinct feature is reversible self-assembly: ferritin cage can be first disassembled under extremely acidic conditions (pH ≤ 2.0); reassembly of ferritin can then occur when the solution pH is adjusted to a neutral range (e.g., pH 7.0). Given the nanosized inner cavity and the reversible assembly characteristic of the ferritin cage, food nutrient molecules, such as β-carotene, epigallocatechin gallate (EGCG), and rutin, have been successfully encapsulated into the phytoferritin or recombinant ferritin (Chen et al., 2014, Yang et al., 2015, Yang et al., 2016). After encapsulation, these nutrient molecules can be functionalized using the ferritin tool to achieve stabilization, solubilization, and targeted delivery. However, the extremely acidic condition at pH 2.0 during ferritin disassembly may cause the loss of a large fraction of the sample to insoluble aggregates. After pH transition (2.0–7.0), the structural integrity of the reassembled ferritin remains undetermined (Tetter & Hilvert, 2017). Moreover, the extremely acidic condition may abrogate the sensory properties, stability, and bioactivity of certain pH-sensitive molecules. Thus, the successful encapsulation of bioactive molecules in the ferritin cage while maintaining the integrity of the ferritin structure presents challenges.
The present study aims to evaluate the effect of ACP treatment on the structure and property of ferritin. The morphology, secondary structure, thermal stability, iron oxidative deposition, and iron release properties of ACP-treated apoRBF (ACPF) were evaluated. The disassembly-reassembly feature of the ACPF was emphasized. We also encapsulated the curcumin molecules into the ACPF. This study is designed to expand the application of ACP to include protein modification and to elucidate the effect of ACP on the structure and property of phytoferritin.
Section snippets
Preparation of apoRBF and analyses of native-PAGE and SDS-PAGE
RBF and apoRBF (RBF deprived of iron ions) were prepared as reported (Li, Yun, Yang, & Zhao, 2013). The molecular weight of apoRBF was determined by native-PAGE using 12% polyacrylamide gradient gels. The running conditions were as follows: 5 mA, 8 h, and 4 °C. SDS-PAGE was performed under reducing conditions in 15% SDS-PAGE. The concentrations of ferritin were determined according to the Lowry method, with bovine serum albumin as a standard sample.
ACP treatment of apoRBF
ACP was performed using a dielectric barrier
Characterization of ACP treated apoRBF by electrophoresis and TEM
In this study, ACP was used to treat apoRBF. SDS-PAGE results showed that the ACP-treated apoRBF consisted of one kind of subunit with a molecular weight of about 28.0 kDa, which was similar to that of apoRBF (Fig. 1a). This result suggested that the ACP treatment did not change the subunit size of apoRBF. Native-PAGE analysis also indicated a single electrophoretic band with a molecular weight similar to that of apoRBF (560 kDa) (Fig. 1b). This result confirmed that the ACP treatment retained
Conclusion
This study was the first to apply ACP treatment to dispose of phytoferritin. The ACP-treated apoRBF retained the typical cage-like structure of ferritin, whereas the contents of the α-helix/β-sheet secondary structure, transition temperature, and surface hydrophobicity of ferritin were decreased. In addition, iron oxidative deposition and iron release activities markedly changed. Notably, the ACP-treated apoRBF could be disassembled into subunits under a benign condition at pH 4.0 and then
Acknowledgements
This project was supported by the Foundation (No. xnc201710) of Tianjin University of Science and Technology, Institute for New Rural Development, P.R. China, the National Natural Science Foundation of China (No. 31501489, 31501544), P.R. China, and Natural Science Foundation of Tianjin City (No. 17JCQNJC14400), P.R. China.
Conflict of interest
The authors have no conflicts of interest to declare.
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