MALDI mass spectrometry of cells, extracts and crude mixtures

by Lay, Jackson O.; Gidden, Jennifer; Liyanage, Rohana

MALDI is usually not applied to mixts. because of problems with selective ionization, suppression, and the interpretation of complex spectra. Nevertheless a no. of applications are well suited for direct MALDI. For example, problems requiring the development of a "fingerprint", such as bacterial taxonomy or differentiation of food lipids are instances where direct MALDI can be useful. Sometimes MALDI can be successfully employed for an initial screen based on anal. of only the abundant or polar species. When rapid anal. if required, direct MALDI is worth consideration because of its simplicity and speed. We have focused on direct MALDI of intact cells, crude mixts., and complex samples after minimal processing. MALDI can be used directly on intact cells to produce a fingerprint that provides genus, species and strain specificity based peptides and small proteins readily desorbed from intact cells. Small recombinant proteins can sometimes be detected directly from the cells. After a brief extn. of bacteria, MALDI of the crude org. layer shows a polar lipid profile complementary to the FAMEs profile. Lipids give the same sort of taxonomic specificity obsd. from peptides and proteins. In some cases anal. of the lipids provides insights lost based on anal. of the fatty acids only. For example, in preliminary expts. with a chicken model, Vitiligo produced changes in the lipids rather than the fatty acids of the dysfunctional melanocytes before they disappeared. We have also used direct MALDI to screen artifacts extd. using minimally destructive SFE procedures. Surprisingly, the SFE ext. of the wrappings of an Egyptian mummy showed evidence for polymn. of glycerol, which could not be attributed to the extn. procedure itself. This finding demonstrates the advantages of applying direct MALDI to crude exts., even when traditional methods like GC/MS are expected to produce the most useful results.