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[ 92 ]. Examination of the two exochelins—from M. neoaurum and Mycobacterium
ADM8563—showed that they had similar properties and were possibly identical.
The presence of a chloroform-soluble carboxymycobactin was not investigated in
this work but it would not have been present in the preparations used because these
had been purified by ion exchange chromatography which would have excluded the
carboxymycobactins. The rate of uptake of iron into M. leprae was, as might have
been expected, very slow but, as exochelin-mediated iron uptake did not occur in
ADM cells that had been grown iron sufficiently, it was concluded that this result
might indicate that M. leprae had been growing iron-deficiently in its host animal
(the armadillo) in order for iron uptake to have taken place at all. It was also sug-
gested that one of these exochelins might be useful additions to any growth medium
that might be being developed for the possible growth of M. leprae in the laboratory.
This however was never additionally studied and the cultivation of M. leprae in cul-
ture medium still remains a distant prospect.
2.5.3 The Carboxymycobactins
The name 'carboxymycobactin' was not given to the extracellular siderophores
that had been isolated from pathogenic species of mycobacteria, including M.
tuberculosis, M. bovis and M. avium and related species, before their structures
had been established which was not until 1995. Up to that date, they were referred
to as the chloroform-soluble exochelins. Perhaps, with hindsight an alternative
name to 'exochelin' might have been used to avoid confusion with the obviously
different water-soluble exochelins that were described in the previous section. But
we (as all the work had been done in the author's laboratory) simply had no idea
what might be their structures. To call the material 'exomycobactin', as was belat-
edly suggested by another group, presupposed we knew it was related to myco-
bactin itself. For all we knew, it could have been based on a completely different
type of structure. What made matters worse, was that initial analysis of the sidero-
phores from the pathogenic species showed that they were composed of a variety
of materials. Barclay and Ratledge [ 48 , 99 ] analyzed the exochelins from species
of mycobacteria belonging to the tuberculosis group, including both H37Rv (viru-
lent) and H37Ra (avirulent) strains of M. tuberculosis , and to the avium groups
using both high performance thin layer chromatography (HPTLC) and high per-
formance liquid chromatography (HPLC). Multiple spots or peaks were revealed;
in some cases upwards of 15 individual compounds could be seen. This was com-
pletely puzzling and suggested that, as there was no large single entity, determin-
ing the structures might be a long and daunting task if each component had to be
isolated and purified. If only we had used a less discriminating technique, such as
ordinary thin layer chromatography, we might have seen just a single spot on the
chromatograms that would then have encouraged us to have the material examined
without delay. But a multiple of spots on HPTLC and peaks with HPLC suggested
there might be a multiple of structures.
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