In an experimental setting, Fc and Fab fragments can be generated in the laboratory. The enzymepapain can be used to cleave an immunoglobulin monomer into two Fab fragments and an Fc fragment. Conversely, the enzyme pepsin cleaves below the hinge region, so the result instead is a F(ab')2 fragment and a pFc' fragment. Recently another enzyme for generation of F(ab')2 has been commercially available. The enzyme IdeS (Immunoglobulin degrading enzyme from Streptococcus pyogenes, trade name FabRICATOR) cleaves IgG in a sequence specific manner at neutral pH. The F(ab')2 fragment can be split into two Fab' fragments by mild reduction.[1]
Heavy and light chains, variable and constant regions of an antibody.
An antibody digested by papain yields three fragments: two Fab fragments and one Fc fragment
An antibody digested by pepsin yields two fragments: a F(ab')2 fragment and a pFc' fragment
The variable regions of the heavy and light chains can be fused together to form a single-chain variable fragment (scFv), which is only half the size of the Fab fragment, yet retains the original specificity of the parent immunoglobulin.[2]
Fab antibodies also have diagnostic use. Arcitumomab is a mouse antibody that recognizes carcinoembryonic antigen, an antigen over-expressed in 95% of colorectal cancers. It is conjugated to a radioactive element, which will label the tumors when viewed with single-photon emission computed tomography. Sulesomab, an antigen that recognizes proteins on the surface of granulocytes, is used to label out infections, again using the 99mTc isotope.[7]
Fab fragments are often fused to small proteins (<100 kDa) that have lower scattering, resulting in images with less contrast.
↑Seger D, Kahn S, Krenzelok EP (2005). "Treatment of US crotalidae bites: comparisons of serum and globulin-based polyvalent and antigen-binding fragment antivenins". Toxicol Rev. 24 (4): 217–227. doi:10.2165/00139709-200524040-00002. PMID16499404. S2CID44916236.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑Fachinformation Lucentis. Novartis Pharma. Stand 15. November 2007.
↑W. J. Köstler, C. C. Zielinski (November 2000). "Diagnostische und therapeutische Antikörper in der Onkologie — State of the Art". Acta Chirurgica Austriaca. 32 (6): 260–263.
