Contents - Faperta

Biosafety of Food from Genetically Modifi ed Crops 449
TABLE 15.2
No Observable Effect Level (NOEL) for Mortality of Rats Following Exposure to Purifi ed
Bt Proteins
Event/Transgene NOEL (mg kg 1 )
Digestibility in Simulated
Gastric Juices
Homology to Known
Food Allergens
Mon810 (Cry1Ab) 4000 Rapidly degraded None
Bt 11 4000 Rapidly degraded None
Cry1F 5080 Rapidly degraded None
Bt 176 (Cry1Ab) 3280 Rapidly degraded None
Cry9C 3760 Stable None
Bt spray 5050 Stable None
Source: http://www.epa.gov/appbppdl/biopesticides/ai/plant-pesticides.html.
The plant-expressed Cry1Ab protein has not shown any toxic effects on rats (Table 15.2). The
levels of Bt protein produced in the currently registered genetically modifi ed plants are quite
low (0.05 μg g 1 of plant tissue). A person would have to consume 27.5 kg of corn to reach
an equivalent dose of 5000 mg protein kg 1 of rat body weight. There is no production of
secondary metabolites due to pleiotropic effects (Pedersen, Eriksen, and Knudsen, 2001), and
there is no change in the products of chymotrypsin inhibitors (Novak and Haslberger, 2000).
The Bt proteins are rapidly degraded by the stomach juices of vertebrates. Most of the
Bt toxins are specifi c to insects as they are activated in the alkaline medium of the insect
gut. Processing removes 97% of the active proteins in transgenic cottonseed (Sims and
Berberich, 1996). CryIAb protein as a component of post-harvest transgenic maize plants
dissipates readily on the surface of, or cultivated into, soil (Sims and Holden, 1996), and
has not been detected in silage prepared from transgenic plants (Fearing et al., 1997). The
nptII gene used as a selectable marker is also readily degraded like other dietary proteins,
and does not compromise the effi cacy of aminoglycoside antibiotics, and presents no risk
to humans (Fuchs et al., 1993). The Bt proteins Cry34Ab1/Cry35Ab1 conferring resistance
to western corn rootworm, Diabrotica virgifera virgifera LeConte, in maize are rapidly
degraded in simulated gastric fl uid, comparable to other registered plant-incorporated
protectants (Herman et al., 2003).
Histopathological effects of Bt proteins have been observed in the gut mucosa, but no
systemic effects have been observed in mice and rabbits following oral administration.
Transgenic Bt tomatoes pose no additional risk to human and animal health (Noteborn
et al., 1996). In vivo and in vitro experiments involving gastrointestinal tissues from rodents,
rhesus monkey, and humans have indicated the absence of specifi c binding sites for Cry1Ab
protein (Noteborn et al., 1995). Short-term toxicity testing revealed no adverse effects in
laboratory animals, and no evidence was found for immunotoxicity of the protein. There
were no apparent differences in percentages of testicular cell populations (haploid, diploid,
and tetraploid) between the mice fed the Bt-corn diet and those fed on conventional corn diet
(Brake, Thaler, and Evenson, 2004). Because of the high rate of cell proliferation and extensive
differentiation that makes testicular germ cells highly susceptible to some toxic agents,
it was concluded that the Bt corn diet had no measurable or observable effect on fetal, postnatal,
pubertal, or adult testicular development, and therefore, Bt corn is not harmful to
human reproductive development.
The fate of Cry1Ab protein has also been examined in the gastrointestinal contents and
visceral organs of calves fed insect-resistant genetically modifi ed maize Bt11. Trace amounts