Cry3Bb1 protein from B. thuringiensis was introduced into corn plants (YieldGard®* Rootworm Corn) for protection from damage by corn rootworm larvae. The mode of action of Cry3Bb1 is well described. The protein is a 8-endotoxin that binds to receptors on brush-border epithelia in the insect midgut and forms ion channels.52 Eventually colloid osmotic lysis kills the cell, as demonstrated in insect-cell culture.53 The receptors define the specificity of the Cry proteins toward insect pests, and are not present in mammals. The Cry3Bb1 protein is selectively toxic to Coleopteran species, with the highest activity against southern corn rootworm (Diabrotica undec-impunctata). The crystal structure of the Cry3Bb1 protein has been determined and described.54 The Cry3Bb1 protein is also contained in the topically applied commercial microbial product, Raven Oil Flowable Bioinsecticide. Microbial pesticides containing B. thuringiensis Cry proteins have been used for more than 45 years and have endured extensive toxicity testing showing no adverse effects to human health.45 Therefore, the Cry3Bb1 protein introduced into corn has a long history of safe use. Bioinformatic analysis comparing the amino acid sequence of the Cry3Bb1 protein to the amino acid sequences of all known allergens and toxins demonstrated the lack of structurally relevant similarities between the Cry3Bb1 protein and any known allergenic, toxic, or pharmacologically active proteins that may adversely impact human or animal health.
The Cry3Bb1 protein is expressed at low levels in corn grain, representing approximately 0.007% (70 ppm) of grain fresh weight. For safety evaluation purposes, a large amount of Cry3Bb1 protein was purified from E. coli cultures expressing Cry3Bb1 and a small amount of protein was purified from corn grain. The equivalence of both proteins was established by demonstrating that the proteins from each source had identical molecular weights, equal immunoreactivities with Cry3Bb1-specific antibodies, they were not glycosylated, and had equivalent functional activities.55 Consequently, the bacteria-produced Cry3Bb1 protein was utilized for safety testing.
Stability of Cry3Bb1 protein to digestion was assessed in an in vitro digestibility assay in SGF containing pepsin. The Cry3Bb1 protein was rapidly (<15 seconds) digested when incubated in SGF, indicating that this protein is unlikely to induce allergenic reactions.56 Cry3Bb1 was tested in an acute oral mouse gavage at a high dose of 3200 mg/kg of BW. When administered at this dose, no evidence of treatment-related adverse effects were observed, hence this dose established
* YieldGard® Rootworm Corn is a registered trademark of Monsanto Technology, LLC. Copyright 2008 by Taylor and Francis Group, LLC
the no-observable-effect-level (NOEL).18 Based on the potential UK adult dietary exposure to corn grain (see Chapter 11) of 0.23 g/kg BW/day multiplied by the amount of Cry3Bb1 protein in YieldGard® corn grain of 0.07 mg/g, a safety margin of approximately 200,000 (3200 mg/kg - 0.016 mg/kg) is calculated, assuming that no Cry3Bb1 protein is lost during processing of corn and that 100% of the daily consumed corn is derived from YieldGard® corn.57 Taken together, these data provide convincing evidence that there is virtually no risk to human and animal health associated with dietary exposure to Cry3Bb1 protein.
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