Progress on the Origin and Formation Mechanism of Semicarbazide in Crustacean Aquatic Products

Nitrofurazone is a synthetic antimicrobial drug developed by the Eaton Institute in the United States in the 1950s. Nitrofurazone can play an inhibitory or bactericidal role by interfering with the glucose metabolism process and oxidase system in bacteria. Due to its strong bactericidal ability, wide antibacterial spectrum, and low price, it was widely used in animal husbandry and aquaculture. Nitrofurazone is detected in animals because it is rapidly metabolized, with a half-life of only a few hours. Semicarbazide (a typical metabolite of nitrofurazone) is detected in food-borne products in a linear proportion to the amount of nitrofurazone added to the animal. Semicarbazide binds to animal proteins to generate stable residues and is difficult to metabolize completely. The United States, European Union, China, and other countries detect and monitor semicarbazide as a marker of nitrofurazone drugs. Nitrofurazone (and its metabolite semicarbazide) have teratogenic and carcinogenic effects on the human body. Any residues in animal-derived foods can be transmitted to humans through the food chain. Long-term intake of semicarbazide in humans will cause anemia, liver necrosis, neuritis, and damages the eyeball and DNA. Therefore, the United States, the European Union, and other countries have explicitly banned its use in the food industry. China has listed nitrofurazone as a banned drug and specified that nitrofurazone and its metabolites should not be detected in animal-derived foods. Over the years, the detection of semicarbazide has been limited by the detection methods and instruments. The Ministry of Agriculture has stated the residual limit of semicarbazide as 1.0 μg/kg and assigned a supervision and sampling inspection program.Existing studies have identified the semicarbazide detected in crustacean aquatic products combines the residue caused by nitrofurazone metabolism and other obvious sources of semicarbazide, which include: 1) the presence of endogenous sources in crustacean aquatic animals; 2) the growth environment and feed intake; and 3) aquatic product processing. Previously, semicarbazide residues were generally considered to be the result of excessive nitrofurazone drug use by farmers. In recent years, the farmers state they have not used nitrofurazone during aquaculture. However, semicarbazide has been present in seafood. In 2004, Saari et al. detected semicarbazide in Procambarus clarkii that did not consume nitrofurazone and provided the first report that crustaceans may naturally produce semicarbazide, which is causing the detection of semicarbazide in many cultured crustacean aquatic animals that have not been fed nitrofurazone drugs (represented by shrimp and crab). This research confirms the presence of endogenous semicarbazones in crustacean aquatic products. In addition, the natural living environment of crustacean aquatic animals is polluted with semicarbazide due to economic human activities. Many scientists have detected the presence of semicarbazide in the waters and sediments in various regions. Concurrently, semicarbazides also contaminate aquatic plants. Semicarbazide is a new water pollutant that exists in water bodies and plants, which is continuously enriched and enters organisms. Nitrofurazone is a commonly used antibiotic for aquaculture products and is often detected when the amino residues exceed the standard levels due to illegal addition by farmers. Studies have shown that semicarbazide is also introduced through processing aquatic products, such as sodium hypochlorite disinfection resulting in an increase in the levels of semicarbazide, by azodicarboxamide through thermal decomposition producing semicarbazide and so on. The biological toxicity of semicarbazide and the food chain transfer effect have ensured semicarbazide is now an important environmental and food pollutant.In the current aquatic trade in China, the presence of endogenous semicarbazide in crustacean aquatic products has serious impacts and interferes in the detection of nitrofurazone drugs, resulting in an inability to accurately determine semicarbazide sources. It is of great importance to thoroughly analyze and understand the main sources and formation mechanism of SEM in crustacean aquatic products to ensure the healthy development of the aquaculture industry in China. At present, there are two statements on the formation mechanism of endogenous semicarbazide: arginine is involved in the urea cycle of crustacean aquatic animals and semicarbazide is produced through the oxadine intermediate. An analysis of content changes in the main substances of the urea cycle revealed the formation of endogenous semicarbazide is closely related to the guanidinyl and amide groups of arginine, citrulline, and the amide structure of urea. Arginine is a potentially important factor in the formation of endogenous semicarbazide; secondly, SEM is derived from a single cell epidermis that produces chitin. There is a single cell epidermal layer secreting chitin between the shrimp shell and shrimp meat, and the detection level of semicarbazide in the shrimp meat close to this epidermal layer was more than three times higher than the inner shrimp meat. Therefore, the semicarbazide in shrimp meat mainly originates from the epidermal layer cells producing chitin. Two inferences on the formation mechanism of exogenous semicarbazide are: the carbamate ions in hypochlorite solution may react with ammonia or acid amide in aquatic products to generate hydrazine, and hydrazine reacts with urea and other compounds through the urea cycle to generate semicarbazide, increasing the production of semicarbazide; the azodicarbonamide added in processing is degraded to biurea at high temperatures, and biurea is then converted to semicarbazide by the hydrolysis reaction. Considering the different molecular structures between nitrofurazone and biurea, the speculation that nitrofurazone is metabolized to produce biurea can be ruled out. From existing studies, azodicarbonamide is the only biological source of biurea, so biurea can be used as the corresponding target detector of azodicarbonamide. To solve the problem that endogenous and exogenous semicarbazide cannot be distinguished in aquatic products in China, the endogenous and exogenous pathways of semicarbazide and the corresponding possible formation mechanisms are reviewed in this paper. The formation pathways of endogenous semicarbazide are speculated to help solve the formation mechanism of semicarbazide in crustacean aquatic products and provide scientific data for the standardization of semicarbazide residue limits in China.

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