Study On Distribution And Speciation Of Cadmium In Rice(Oryza Sativa L.)

China is the largest producer and consumer of rice(Oryza sativa L.)in the world.Heavy metal residues in soils caused by industrial and environmental pollution have become an important risk factor in the rice industry.In order to solve the problem of cadmium pollution in rice,many methods such as soil remediation and improvement of rice varieties have been developed.However,limited by the high cost and long treatment period of the above methods,cadmium-contaminated rice were still produced,and threatens the health of consumers.Besides boiled or steamed to make staple food,rice can be used to produce rice wine,rice flour,rice bran oil,rice starch,rice proteins,and other foods or food raw materials.Rice processing method is an important way to effectively utilize cadmium-contaminated rice and ensure the quality and safety of foods.In order to develop proper cadmium reduction processing technologies,it is necessary to determine distribution sites and speciation of cadmium in rice grains.Cadmium distribution sites in rice grains were determined by physical separation and laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS).Through separation,extraction,microwave digestion,and inductively coupled plasma-mass spectrometry(ICP-MS)analysis,cadmium-binding proteins in rice grains were explored.The protein fractions,molecular weights,and amino acid compositions of cadmium-binding proteins were determined by Osborne extraction,amino acid analysis,and size exclusion chromatograph-inductively coupled plasma-mass spectrometry(SEC-ICP-MS).The protein was purified by Iminodiaceticacid(IDA)immobilized ion affinity chromatography and size exclusion chromatograph(SEC),the cadmium-binding proteins were analyzed and identified by iTRAQ technique with QE high-resolution mass spectrometry.In addition,we analyzed the cadmium-binding proteins in different organs and grains of rice plants and studied the source and structural specificity of cadmium-binding proteins in rice grains.The main conclusions are drawn as follows:1.In brown rice grains,the distribution of cadmium was uneven.The cadmium content was gradually decreased from the outer layer to the inner organs.Cadmium was mainly distributed in embryo,aleurone layer and outer endosperm.The content of cadmium in the parts between embryo and endosperm was significantly lower than that in embryo and endosperm and close to that in endosperm.2.The protein-binding state is the most important form of cadmium in rice grains and accounts for 53.4%of the total cadmium content in grains.The starch-binding state accounts for 12.3%and other forms account for 34.3%.The cadmium-binding ability of different proteins was decreased according to the following order:globulin>albumin>gluten>gliadin.The contents of two sulfur-contained amino acids(methionine and cysteine)were positively correlated with cadmium content in proteins,suggesting that sulfur-contained residues in the protein group played an important role in the adsorption of cadmium.The specific molecular weights of cadmium-binding proteins were respectively 2000 kDa,572.1 kDa,and 131.9 kDa.3.According to the identification results of cadmium-binding proteins,189 cadmium-binding proteins were detected in rice grains,including 31%of putative uncharacterized proteins,29%of Cupin family proteins,12%nonspecific lipid-transfer proteins,6%allergenic proteins,4%19 kDa globulin,1%Glutelin,and 17%other proteins.These protein were queried and analyzed in Uniprot database,the Cd binding ability were not been reported yet.4.Cadmium-binding proteins in rice roots were large molecular weight proteins,whereas cadmium-binding proteins in rice stem and leaves were low molecular weight proteins.According to the identification results of cadmium-binding proteins from different organs,1225,1881,and 2165 cadmium-binding proteins were respectively found in root,stem,and leaves.The homology analysis of cadmium-binding proteins from different organs showed that cadmium-binding protein sequences were different and the primary structures of cadmium-binding proteins were not conserved.Cadmium-binding specific sequence was not found.