Characterization of cerebrosides from the thermally dimorphic mycopathogen Histoplasma capsulatum: expression of 2-hydroxy fatty N-acyl (E)-Delta(3)-unsaturation correlates with the yeast-mycelium phase transition.

Cerebroside (monohexosylceramide) components were identified in neutral lipids extracted from both the yeast and mycelial forms of the thermally dimorphic mycopathogen Histoplasma capsulatum. The components were purified from both forms and their structures elucidated by 1- and 2-dimensional nuclear magnetic resonance (NMR) spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and low energy tandem collision-induced dissociation mass spectrometry (ESI-MS/CID-MS). Both components were characterized as beta-glucopyranosylceramides (GlcCers) containing (4E,8E)-9-methyl-4,8-sphingadienine as the long-chain base, attached to 18-carbon 2-hydroxy fatty N-acyl components. However, while the fatty acid of the yeast form GlcCer was virtually all N-2'-hydroxyoctadecanoate, the mycelium form GlcCer was characterized by almost exclusive expression of N-2'-hydroxy-(E)-delta(3)-octadecenoate. These results suggest that the yeast-mycelium transition is accompanied by up-regulation of an as yet uncharacterized ceramide or cerebroside 2-hydroxy fatty N-acyl (E)-delta(3)-desaturase activity. They also constitute further evidence for the existence of two distinct pathways for ceramide biosynthesis in fungi, since glycosylinositol phosphorylceramides (GIPCs), the other major class of fungal glycosphingolipids, are found with ceramides consisting of 4-hydroxysphinganine (phytosphingosine) and longer chain 2-hydroxy fatty acids. In addition to identification of the major glucocerebroside components, minor components (< 5%) detectable by molecular weight differences in the ESI-MS profiles were also characterized by tandem ESI-MS/CID-MS analysis. These minor components were identified as variants differing in fatty acyl chain length, or the absence of the sphingoid 9-methyl group or (E)-delta(8)-unsaturation, and are hypothesized to be either biosynthetic intermediates or the result of imperfect chemical transformation by the enzymes responsible for these features. Possible implications of these findings with respect to chemotaxonomy, compartmentalization of fungal glycosphingolipid biosynthetic pathways, and regulation of morphological transitions in H.capsulatum and other dimorphic fungi are discussed.