Apolar Mycosides

It has now been recognized that two groups of mycosides C can be distinguished, the apolar mycosides C and the antigenic polar mycosides C (777). 

Apolar mycosides C were first isolated during a search for substances responsible for an absorption band at 1235 cm from extracts of M. avium 178), M. scrofulaceum (using a strain called M. marianum at that time) (779) and M. butyricum which was probably a strain of M. smegmatis 180). As will be shown all these compounds have the same general structure and differ only in detail. 

Chemical studies performed on these mycosides showed the presence of a common tripeptide backbone D-Phe-D-a//oThr-D-Ala 178,179) sometimes extended to the pentapeptide D-Phe-D-a//oThr-D-Ala-D-a//oThr-D-Ala 178,179). However it must be kept in mind that amino acids linked as amides to fatty acids are difficult to split off by hydrolysis, so their levels are often underestimated. Moreover, when mass spectro-metric measurements were performed, only the tripeptide was found 

Two different sugar molecules were found per molecule of mycoside C. These sugars were identified as 6-deoxytalose and O-methyl derivatives of rhamnose, 2,3,4-tri-O-methylrhamnose and more often 3,4-di-O-methylrhamnose (184). Later on (185) these sugars were said to be 6-deoxy-L-talose and O-methyl derivatives of L-rhamnose, but no experimental evidence for their absolute stereochemistry was provided. 

By partial acid hydrolysis a dipeptide was isolated and identified as d-a//othreonyl-D-alanine. Furthermore, it was observed that mild treatment with alkali cleaved mycoside C to free 6-deoxytalose and a modified mycoside devoid of 6-deoxytalose as well as a/Zothreonine. The concomitant disappearance of 6-deoxytalose and of aZZothreonine suggested P- 

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As the polar mycosides C behave as antigens they must be located in the outer layers of the mycobacterial cell. In fact their external location was demonstrated in M. intracellulare by the use of ferritin-labelled antibodies 214). It has long been known that mycobacteria of the MAIS complex are surrounded by an electron-transparent zone when ultra-thin sections of infected host tissues are examined under the electron microscope. This zone is essentially composed of parallel fibrils wrapped longitudinally around the bacterium. This material was isolated by the use of urea density gradients and analyzed chemically. The three amino acids characteristic of mycosides C and alaninol as well as 6-deoxytalose were identified 194). This material consisted mainly of a mixture of polar and apolar mycosides C, in a ratio about 7 3 (275). 

The structures of the apolar C-mycoside GPLs were established by several groups of French investigators in the period from 1967 to 1971. A moot question for some time was whether the internal -D-alloThr-D-Ala- unit was repeating. However, with the recognition of 2-aminopropanol (alaninol) the now-accepted structure 1 for the lipopeptide core was advanced. Further unequivocal proof of the accepted core structure was provided by field-desorption MS and by californium-... 

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