Mycobacterium avium complex (MAC) is a group of non-tuberculous mycobacteria that cause tuberculosis-like diseases in humans. Unlike Mycobacterium tuberculosis, MAC expresses high levels of glycopeptidolipids (GPLs) containing a well-defined tetrapeptide-amino alcohol core, composed of D-phenylalanine, D-allo-threonine, D-alanine, and L-alaninol, that is modified with a fatty acid and sugar residues. Surprisingly, however, a careful scrutiny of the mass spectrum of MAC GPLs revealed the presence of ions that could not readily accountable for the known GPL structure. The magnitude of the ions was increased prominently when GPLs were isolated from the valine-supplemented culture, and the ions representing the authentic GPL species were diminished, suggesting the possibility that the basic structure of the peptide backbone might be altered in response to the exogenously added valine. Indeed, further mass spectrometry (MS)/MS and gas chromatography-MS analysis indicated a substitution of D-valine for the N-terminal D-phenylalanine of the tetrapeptide core, and the presence of D-valine and the absence of D-phenylalanine was confirmed by high-performance liquid chromatography, using the derivatized amino acid residues that were released from the tetrapeptide. Finally, specific antibodies to the purified valine-containing GPL species were detected in the serum of a MAC-infected guinea pig. Therefore, these results identify a new molecular species of MAC GPLs with immunogenic potential. (C) 2012 Elsevier Inc. All rights reserved.