Peptide quantification by matrix-assisted laser desorption ionisation time-of-flight mass spectrometry: investigations of the cyclotide kalata B1 in biological fluids.

A rapid method has been developed for the quantification of the prototypic cyclotide kalata B1 in water and plasma utilizing matrix-assisted laser desorption ionisation time-of-flight (MALDI-TOF) mass spectrometry. The unusual structure of the cyclotides means that they do not ionise as readily as linear peptides and as a result of their low ionisation efficiency, traditional LC/MS analyses were not able to reach the levels of detection required for the quantification of cyclotides in plasma for pharmacokinetic studies. MALDI-TOF-MS analysis showed linearity (R2 > 0.99) in the concentration range 0.05-10 microg/mL with a limit of detection of 0.05 microg/mL (9 fmol) in plasma. This paper highlights the applicability of MALDI-TOF mass spectrometry for the rapid and sensitive quantification of peptides in biological samples without the need for extensive extraction procedures.