e. breast cancer cells. While the first three types may all express specific binding sites for purified Bt 18 toxin, MCF-7, being a totally different class of cells, may not exhibit A-1210477 mouse similar binding sites for the toxin. Since comparisons had already been made between CEM-SS and two other leukaemic cell lines (CCRF-SB and CCRF-HSB-2), MCF-7 was used in this case to demonstrate that a different class of cell line
may show lower affinity for the purified toxin. When compared to learn more experiments performed previously, the binding results agreed well with the cell viability assays. Purified Bt18 toxin exhibits cytotoxocity towards CEM-SS cells whereas MCF-7 cells are relatively unharmed [17]. The lower cytotoxicity of the toxin for MCF-7 cells may be explained by the lower affinity the toxin has for these cells. The scarcity of literature for the binding mechanisms of parasporin makes comparison of binding affinity of purified Bt 18 toxin on CEM-SS with other Bt parasporal proteins and cancer cell types difficult. However, from binding experiments done on insects, it was found that the dissociation constants of various Bt toxins for insect cells were higher than that of purified Bt 18 toxin for CEM-SS cells. As the dissociation GSK621 clinical trial constant is inversely proportional to the binding affinity, this implies that binding affinity of purified Bt 18 toxin for CEM-SS cells was relatively higher than that of other Bt toxins for insect cells
[18, 19]. This finding is interesting as it may mean that the weak cytotoxicity of purified Bt 18 toxin on leukaemic cells could be influenced by factors other than its binding affinity for Org 27569 the cell line since the binding affinity was found to be relatively higher in comparison with insect studies. Heterologous competitive binding assays suggested that there was a minor degree of competition between biotinylated Bt 18 toxin and crude Btj toxin as well as crude Bt
22 toxin as the percentage of bound biotinylated toxin was significantly decreased to 78% (p < 0.001) and 80.81% (p < 0.05) at 59.26 nM respectively. This low degree of competition might or might not represent true competition among toxins because it was also observed that at such concentration, there was a significant cell death of 10.66% (p < 0.05) and 2.65% (p < 0.05) for crude Btj toxin and crude Bt 22 toxin respectively (results not shown). The decrease in the percentage of the bound biotinylated toxin might be confounded by cell death that occurred at the same time. Besides, it may also be confounded by the possibility of non-specific binding sites. However, even if true competition were to occur, the degree of competition was small as only approximately 20% displacement of the biotinylated toxin occurred for both crude Btj toxin and crude Bt 22 toxin. Little or no competition between biotinylated purified Bt 18 toxin and crude Btj toxin further supported earlier results by Nadarajah et al.