Can you comment on the mechanism by which the SAP toxin is cleaved off from the antibody? Your website indicates that the cleavage occurs in the endosome. I just want to verify that it is not cleaved in the lysosome.
From what we have gathered, there is a great probability that something gets broken in the endosome. We know this from the peptide ligand toxins that bind to GPCRs. They would be rapidly returned to the cell surface through receptor recycling if there wasn’t some sort of cleavage. In the case of FGF-SAP (Cat. #IT-38) , e.g., we know that FGF is extensively degraded in the endosome through proteolytic degradation (Lappi et al, 1994). There is occasionally a disulfide linker between the toxin and antibody, but there is some controversy that this is cleaved: many say yes, some say no, mainly because the redox potential is not sufficient. This would ignore the presence of thiol reductase enzymes. The single chain antibody fusion protein-toxins are quite toxic. The linkage there is clearly through peptide bonds (they are fusion proteins) so the easiest response to this question is that there is proteolytic degradation. Since saporin is tremendously resistant to proteases, it can’t be stopped.
Lappi DA, Ying W, Barthelemy I, Martineau D, Prieto I, Benatti L, Soria M, Baird A (1994) Expression and activities of a recombinant basic fibroblast growth factor-saporin fusion protein. J Biol Chem 269(17):12552-12558. doi: PMID: 8175664