De novo sequencing of two new cyclic theta-defensins from baboon (Papio hamadryas) leukocytes by matrix-assisted laser desorption/ionization mass spectrometry.

Two cyclic theta-defensin peptides were isolated from leukocytes of the hamadryas baboon, Papio hamadryas, and purified to homogeneity by gel electrophoresis and reversed-phase high-performance liquid chromatography. Both peptides had high in vitro activity against Escherichia coli, Listeria monocytogenes, methicillin-resistant Staphylococcus aureus (MRSA) and Candida albicans. Here, we report their de novo sequencing by matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry (MALDI-TOF/TOF-MS). This was accomplished by combining conventional enzymatic digestion with N-terminal derivatization by 2-sulfobenzoic acid cyclic anhydride (SACA) or 4-sulfophenylisothiocyanate (SPITC) to facilitate the interpretation of fragment ion spectra. In addition to the two cyclic theta-defensins (PhTDs) we also sequenced a novel Papio hamadryas alpha-defensin, PhD-4, which showed high sequence homology to rhesus alpha-defensin RMAD-1 and human alpha-defensin HNP-1.