Peptide Backbone Modifications for the Assessment of Cleavage Site Relevance in Precursors of Signaling Peptides.

The physiological relevance of site-specific precursor processing for the biogenesis of peptide hormones and growth factors can be demonstrated in genetic complementation experiments, in which a gain of function is observed for the cleavable wild-type precursor, but not for a non-cleavable precursor mutant. Similarly, cleavable and non-cleavable synthetic peptides can be used in bioassays to test whether processing is required for bioactivity. In genetic complementation experiments, site-directed mutagenesis has to be used to mask a processing site against proteolysis. Peptide-based bioassays have the distinctive advantage that peptides can be protected against proteolytic cleavage by backbone modifications, i.e., without changing the amino acid sequence. Peptide backbone modifications have been employed to increase the metabolic stability of peptide drugs, and in basic research, to investigate whether processing at a certain site is required for precursor maturation and formation of the bioactive peptide. For this approach, it is important to show that modification of the peptide backbone has the desired effect and does indeed protect the respective peptide bond against proteolysis. This can be accomplished with the MALDI-TOF mass spectrometry-based assay we describe here.