Limonin, the main bioactive phytochemical constituent of limonoids with multi-functions, is enriched in citrus fruits and often found at a high concentration in citrus seeds. The present study was attempted to introduce a new and efficient extraction method to isolate limonoids from pummelo seeds, and to evaluate the antioxidant property of the main constituent limonin in HepG2 cells. Three key single factors were identified for the extraction of limonoids from pummelo seeds using the Box-Behnken experiment design of response surface methodology (RSM), and the optimized extraction parameters were treatment with 89.68 mL of anhydrous acetone for 4.62 h at 78.94 °C, while the yield of limonoids was 11.52 mg/g. The structure of isolated main constituent of the limonoids was further identified as limonin by Fourier transform infrared (FT-IR) spectrometer and nuclear magnetic resonance (NMR) spectrum. Moreover, the molecular data in HepG2 cells revealed that limonin exerted its anti-oxidant property mainly by the activation of nuclear factor (erythroid-2)-like 2 (Nrf2)/kelch-like ECH-associated protein 1 (Keap1)- antioxidant response element (ARE) pathway in the form of transcriptional regulation of Nrf2 mRNA and posttranscriptional regulation of Nrf2/Keap1 system. These results demonstrate that pummelo seeds are an ideal source of limonoids, and limonin is proved to exert its anti-oxidant property by the activation of Nrf2/Keap1 pathway.
The application of organic fertilizers has been suspected as an important source of microplastic (MP) pollution in agricultural soils. However, limited data have been reported regarding the quantity and characteristics of MPs in organic fertilizers, giving rise to the difficulties in their risk assessment. This study investigated the occurrence of MPs in 23 commercial organic fertilizers and 2 farm composts, covering different raw materials and treatment processes. The average MP concentration in the commercial organic fertilizers was 9210.4 ± 1743.1 items/kg, significantly lower than that in farm composts (24344.1 ± 2697.2 items/kg). Among commercial organic fertilizers, the MP loads varied significantly with the source materials and their processing. Organic fertilizers derived solely or proportionally from plant-derived materials through mechanical shredding and sieving had lower MP abundances. The polymer sizes, morphotypes, and colors of MPs in studied samples were mainly < 3 mm, fiber, and black, respectively. The variations in the MP characteristics implied their diverse sources. This study provided a detailed assessment of MP accumulation in organic fertilizers, and confirmed their significance to the MP pollution in terrestrial ecosystem. The results also highlighted the importance of establishing standards to regulate the contents of plastics in raw materials and end products and the treatment processes for the manufacture, thus ensuring the safe application of organic fertilizers.
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