Quantitative analysis of backbone-cyclised peptides in plants

Progress in understanding the biosynthetic pathway of the cyclotides has been hampered as this unique family of cyclic plant peptides are notoriously difficult to analyse by standard proteomic approaches such as gel electrophoresis. We have developed a simple, rapid and robust strategy for the quantification of cyclotides in crude plant extracts using MALDI-TOF MS making use of generic peptides similar in mass to the analyte as internal standards for calibration. Linearity (r(2)>0.99) over two orders of magnitude (down to femtomole levels) was achieved in plant extracts, allowing quantitative analysis of transgenic and endogenous peptide expression.