Some catalytic antibodies have been raised which stabilize the transition state, or the intermediate between the substrate and product of a chemical reaction. Other antibodies that recognize polypeptide chains also happen to proteolytically cleave their targets. First author Srinath Rangan screened 12 such antibodies for their ability to cleave benzyloxycarbonyl-L-lysine o-nitrophenol ester (Z-lys-o-Np), a synthetic peptide that is also a substrate for α-secretase. An enzyme or antibody that cuts Z-lys-o-Np may also cleave amyloid-β (Aβ) at its α-secretase site between residues 16 and 17, and potentially reduce amyloid burden.

Using the synthetic peptide, Rangan and colleagues found that two of the 12 antibodies, dubbed c23.5 and hk14, had α-secretase-like activity. To test if they could also cleave Aβ, the authors incubated them with Aβ40, then tested the solution for degradation products by mass spectroscopy. C23.5 was found to yield two major products corresponding to amino acids 1-16 and 17-40 of Aβ, indicating that the amyloid peptide had indeed been cleaved at the α-secretase site. In contrast, hk14 generated a range of polypeptides with sizes consistent with carboxypeptidase activity.

The use of light chain antibodies to remove or destroy Aβ may circumvent the inflammatory problems that could be encountered with full-length antibodies, but the catalytic approach is not without its own drawbacks. While the α-secretase activity of c23.5 is similar to values published for other proteolytic antibodies, it is not that high. In addition, as the authors write, c23.5 suffers from a specificity problem, in that it can cleave other proteins, including vasoactive intestinal peptide. The success of such antibodies as therapeutics "will depend on the success of future antibody engineering studies to increase the specificity of the parent light chains and to control oligomerization," according to the authors, the oligomerization being that enhanced by the 17-40 amino acid fragment of Aβ, which is more hydrophobic than the full-length peptide.—Tom Fagan.

Reference:
Rangan SK, Liu R, Brune D, Planque S, Paul S, Sierks MR. Degradation of beta-amyloid by proteolytic antibody light chains. Biochemistry. 2003 Dec 9;42(48):14328-34. Abstract