Metabolites in islets of Langerhans and Escherichia coli strain DH5-alpha were 
analyzed using negative-mode, matrix-assisted laser desorption/ionization 
time-of-flight mass spectrometry (MALDI-TOF-MS). For analysis of anionic 
metabolites by MALDI, 9-aminoacridine as the matrix yielded a far superior 
signal in comparison to alpha-cyano-4-hydroxycinnamic acid, 2,5-dihydrobenzoic 
acid, 2,4,6,-trihydroxyacetophenone, and 3-hydroxypicolinic acid. Limits of 
detection for metabolite standards were as low as 15 nM for GDP, GTP, ADP, and 
ATP and as high as 1 muM for succinate in 1-muL samples. Analysis of islet 
extracts allowed detection of 44 metabolites, 29 of which were tentatively 
identified by matching molecular weight to compounds in METLIN and KEGG 
databases. Relative quantification was demonstrated by comparing the ratio of 
selected di- and triphosphorylated nucleotides for islets incubated with 
different concentrations of glucose. For islets at 3 mM glucose, concentration 
ratios of ATP/ADP, GTP/GDP, and UTP/UDP were 1.9 +/- 1.39, 1.12 +/- 0.50, and 
0.79 +/- 0.35 respectively, and at 20 mM glucose stimulation, the ratios 
increased to 4.13 +/- 1.89, 5.62 +/-4.48, and 4.30 +/- 4.07 (n = 3). Analysis 
was also performed by placing individual, intact islets on a MALDI target plate 
with matrix and impinging the laser directly on the dried islet. Direct analysis 
of single islets allowed detection of 43 metabolites, 28 of which were database 
identifiable. A total of 43% of detected metabolites from direct islet analysis 
were different from those detected in islet extracts. The method was extended to 
prokaryotic cells by analysis of extracts from E. coli. Sixty metabolites were 
detected, 39 of which matched compounds in the MetaCyc database. A total of 27% 
of the metabolites detected from prokaryotes overlapped those found in islets. 
These results show that MALDI can be used for detection of metabolites in 
complex biological samples.