Detection of bacterial lipoic acid. A modified gas-chromatographic-mass-spectrometric procedure

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Abstract
The detection of bacterial lipoic acid by a modified g.c.-m.s. procedure is reported. Cells were hydrolysed in HCl to release protein-bound lipoic acid, which, after extraction into benzene, was reduced with NaBH4. The dihydrolipic acid so generated was then isolated by covalent chromatography on dithiolspecific p-aminophenylarsenoxide-agarose and, after elution by 2,3-dimercaptopropane-1-sulphonic acid and extraction into benzene, was allowed to O2-oxidize to the disulphide form. The isolated lipoic acid was allowed to react with diazomethane, and the methyl ester so produced was detected by g.c.-m.s. Analysis of the mass spectrum showed the characteristic molecular ion and seven fragmentation ions, which, along with the identification of those ions retaining the two sulphur atoms, allows the definitive detection of lipoic acid. The methodology has been successfully tested with authentic lipoic acid, the 2-oxoglutarate dehydrogenase multienzyme complex and with whole cells of Escherichia coli. In addition, it has been used to search for and identify lipoic acid in the archaebacterium Halobacterium halobium. The significance of this discovery and the possible roles of the cofactor in H. halobium are discussed.