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3.3 Protein-based methods

Detection methods based on the immunological detection of proteins or on the comparison of protein patterns (e.g. one- or two-dimensional gel electrophoresis) require that the sample or the protein of interest are not significantly degraded. Thus, the application of protein-based detection methods for the identification of genetically engineered food products is generally restricted to fresh (or frozen) and unprocessed foods.

Protein samples obtained from GMOs can be resolved with one-dimensional SDS-gel electrophoresis. Unfortunately, the resolution is not sufficient to clearly distinguish the protein pattern of a GMO from the protein pattern of its conventional counterpart. Two-dimensional gel electrophoresis provides better resolution, but still may generally not be able to provide unequivocal identification of a (trans-) gene product unless combined with immunological methods. The expression level of transgene products in plants were reported to constitute 0 to 2 % of the total soluble protein even when strong constitutive promoters were used to drive expression (Longstaff et al., 1995). Expression levels found in approved genetically engineered crops are generally lower (Approved genetically modified crops in the United States) than the reported upper figure of 2 %. Provided that specific antibodies against the proteins encoded by the transgenes are available, one-dimensional (Padgette et al., 1995; Wood et al., 1995; Yang et al., 1996) and certainly also two-dimensional gel electrophoresis, in combination with Western-blot analysis are suitable detection methods. ELISA can also be an inexpensive but powerful technique (Padgette et al., 1995; Wood et al., 1995). Recently developed techniques using immunosensors have up to now mainly been used for the analysis of serum and blood samples (Morgan et al., 1996). All immunological methods described above, depend on the availability of highly specific antibodies. The latter are commercially available only for a small number of proteins that are the products of transgenes used in approved genetically engineered crops. To our knowledge, these are antibodies against the nptII-gene product, NPTII, or APH(3')II, and against the product of the gus gene. Since the nptII gene is present in 17 of the 28 approved genetically engineered agricultural crops (see section Survey of the structural genes used) and is under the control of a eucaryotic promoter in 16 of these crops (Approved genetically modified crops in the United States and Approved genetically modified crops in the European Community), the development of a screening method based on the immunological detection of NPTII (ELISA-, Dot-Blot-, or Western analysis) may represent an interesting and rather inexpensive possibility.