Identification of a major low-molecular-mass thiol of the trypanosomatid Crithidia fasciculata as ovothiol A. Facile isolation and structural analysis of the bimane derivative.

An unidentified low-molecular-mass thiol, U23, previously detected as the 7-diethylamino-3-(4'-maleimidylphenyl)-4-methylcoumarin derivative in extracts of the trypanosome Crithidia fasciculata, was purified as the bimane derivative. Resonances attributable to U23 were discerned from those of the bimane label by comparison of the 1H- and 13C-NMR spectra of monobromobimane and U23-bimane. The complete 1H- and 13C-NMR spectra of U23-bimane were assigned by means of 1H-1H correlation spectroscopy, 1H-13C correlation spectroscopy and 13C multiplicity determinations. The results indicated identity of U23 with 1-N-methyl-4-mercaptohistidine (ovothiol A), previously isolated from marine sources. This assignment was confirmed by NOE difference experiments, fast-atom-bombardment mass spectrometry of U23-bimane and ultraviolet/visible spectrophotometry of U23, which was isolated as the disulfide. The isolation of ovothiol A from a parasitic protozoan suggest that the 4-mercaptohistidines may have a wider distribution and function as antioxidant thiols than was hitherto realized.