Bt Research 2025, Vol.16, No.5, 214-223 http://microbescipublisher.com/index.php/bt 215 2 Physical and Chemical Properties and Environmental Behavior of Bt Toxins 2.1 Structural characteristics and stability of Bt toxin Bt toxin usually exists in crystal form, with a molecular weight of about 130~140 kDa, and contains three functional domains: Domain I is a seven-helical bundle structure, which is involved in the perforation of pores on the cell membrane of the worm and gastric epithelial cell; Domain II and Domain III are folded structures, which are responsible for receptor binding and crystal stability, respectively. The Bt toxin in the natural state is a protoxin protein. It only shows toxicity after being dissolved in alkaline conditions and cleaved and activated by insect midgut protease. Bt toxin crystals are relatively stable under pure dry conditions and have certain tolerance to high temperatures, but their insecticidal activity is more sensitive to ultraviolet rays and extreme pH. For example, UV-B rays in sunlight can rapidly inactivate Bt toxin proteins, and the half-life of Bt preparations sprayed in the field often takes less than a few days in the sub-sun light (Zhou et al., 2015). It is reported that under strong light irradiation and high temperature conditions, the degradation rate of Bt toxin is significantly accelerated. In addition, Bt toxin is easily soluble in alkaline solutions, but has low solubility in neutral and partially acidic environments, which means that Bt proteins exist in crystalline or particle states in most soils and water bodies and are not prone to long-distance migration (Zhao et al., 2023). 2.2 Adsorption and conversion characteristics of Bt toxins in different media After Bt toxin enters the environment, a series of physical and chemical processes will occur, among which adsorption is particularly important. In solid-phase media such as soil, Bt proteins are quickly adsorbed and bound by clay mineral particles and humus to form a "bound state" Bt toxin. Studies have shown that Bt toxin can be released into the soil for about 2 hours to achieve adsorption-desorption equilibrium. The adsorption capacity and affinity of Bt toxins in the soil depend on factors such as soil texture and organic matter content. In addition to soil, particulate matter and sediments suspended in water can also adsorb Bt toxins. Farmland runoff may carry Bt toxin proteins into rivers and lakes, where large particles of organic matter and mineral particles will quickly precipitate and adsorb them, thereby reducing the effective concentration of Bt toxins in the aqueous phase. The adsorption process affects the subsequent transformation and degradation of Bt toxins in the environment. The adsorbed Bt protein gradually loses water solubility in the soil, but environmental microorganisms and enzymes can still slowly act on their surface (Helassa et al., 2011). As a nitrogen-containing organic compound, Bt toxin is essentially degraded by microorganisms that hydrolyze the protein into small peptides and amino acids, and then convert it into inorganic nitrogen through ammonization and nitration to enter the soil nitrogen cycle. This biological transformation process is relatively slow, especially when Bt toxin is strongly adsorbed by the solid phase of the soil, the conversion rate is further reduced (Liu et al., 2021). 2.3 Key scientific issues in research on Bt toxin degradation Although a large number of studies have focused on the degradation fate of Bt toxins in the environment, there are still several key scientific issues that need to be deepened. The environmental retention time of Bt toxin reported in different studies varies greatly, and it is necessary to clarify the main factors affecting the degradation rate of Bt toxin and their mechanism of action. Secondly, the multi-path destination of Bt toxins in complex environments needs to be comprehensively investigated. For example, in farmland ecosystems, part of Bt protein is degraded by the soil, and part of it may enter the water body with runoff or enter the food web through biological action. What is the overall quality income and expenditure? Again, the intermediate products and their ecological effects in the degradation process of Bt toxin are worthy of attention. There are few studies on whether the small peptides and amino acids produced after the degradation of Bt protein have residual biological activity and whether they have an impact on soil nitrogen circulation or soil organisms. The methods and standards for research on Bt toxin degradation in different environmental matrixes need to be unified. The Bt protein detection methods and sampling frequency used in each study vary greatly, resulting in limited comparability of the results. Therefore, establishing a standardized Bt toxin environmental residue detection and degradation kinetics model is particularly important for the comparison and integration of different research results.
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