Plasma-desorption mass spectrometry of intact enzymes and proenzymes

The plasma desorption (PD) mass spectra of several enzymes and proenzymes are reported. Chymotrypsin and chymotrypsinogen both yield usable PD mass spectra; however, the single-chain chymotrypsinogen exhibits better sensitivity and less fragmentation than the multi-chain chymotrypsin. Porcine pepsinogen contains 11 more basic residues (lysine and arginine) than porcine pepsin, but this does not lead to an increase in positive ion response. In fact, the additional basic residues may hinder response by increasing the strength of the interaction between the protein and the nitrocellulose surface. A series of subtilisins showed comparable PD mass spectrometric, response despite the fact that they differ considerably in primary sequence. Finally, PD mass spectrometry was used to correct the previously reported mass of a heat-stable protease, a value that we found to be in error by almost 5000 Da.