Characterization of bifunctional sphingolipid Δ4-desaturases/C4-hydroxylases of trypanosomatids by liquid chromatography-electrospray tandem mass spectrometry.

Six genes encoding putative sphingolipid desaturases have been identified in trypanosomatid genomes: one in Trypanosoma brucei (TbSLdes protein), one in Trypanosoma cruzi (TcSLdes) and four in Leishmania major (LmSLdes1-4), tandemly arrayed on chromosome 26. The six amino acid sequences showed the three characteristic histidine boxes, with a long spacer between the first and second box, as in fungal desaturases and bifunctional desaturases/hydroxylases, to which they are phylogenetically related. We functionally characterized the trypanosomatid enzymes by their expression in Saccharomyces cerevisiae sur2Δ mutant, which lacks C4-hydroxylase activity. The sphingoid base profile (dinitrophenyl derivatives) of each yeast mutant transformed with each one of the different parasite genes was analyzed by HPLC, using a sur2Δ mutant expressing the Schyzosaccharomyces pombe sphingolipid desaturase (SpSLdes) as positive control. TbSLdes was capable of desaturating endogenous sphingolipids at levels comparable to those found in SpSLdes. By contrast, L. major and T. cruzi enzymes showed either no or negligible activities. Using the HPLC system coupled to electrospray tandem quadrupole/time of flight mass spectrometry we were able to detect significant levels of desaturated and hydroxylated sphingoid bases in extracts of all transformed yeast mutants, except for those transformed with the empty vector. These results indicate that S. pombe, T. brucei, T. cruzi and L. major enzymes are all bifunctional. Using the same methodology, desaturated and hydroxylated sphingoid bases were detected in T. cruzi epimastigotes and L. major promastigote cells, as described previously, and in T. brucei procyclic and bloodstream forms for the first time.