BIOSYNTHETIC ORIGIN OF MYCOBACTERIAL CELL-WALL ARABINOSYL RESIDUES

Designing new drugs that inhibit the biosynthesis of the D-arabinan moiety of the mycobacterial cell wall arabinogalactan is one important basic approach for treatment of mycobacterial diseases, However, the biosynthetic origin of the D-arabinosyl monosaccharide residues themselves is not known, To obtain information on this issue, mycobacteria growing in culture were fed glucose labeled with C-14 or H-3 in specific positions, The resulting radiolabeled cell walls were isolated and hydrolyzed, the arabinose and galactose were separated by high-pressure liquid chromatography, and the radioactivity in each sugar was determined, [U-C-14]glucose, [6-H-3]glucose, [6-C-14]glucose, and [1-C-14]glucose were all converted to cell wall arabinosyl residues with equal retention of radioactivity, The positions of the labeled atoms in the arabinose made from [1-C-14]glucose and [6-H-3]glucose were shown to be C-l and H-5, respectively, These results demonstrated that the arabinose carbon skeleton is formed via the nonoxidative pentose shunt and not via hexose decarboxylation or via triose condensations, Since the pentose shunt product, ribulose-5-phosphate, is converted to arabinose-5-phosphate as the first step in 3-keto-D-manno-octulosonic acid biosynthesis by gram-negative bacteria, such a conversion was then searched for in mycobacteria, However, cell-free enzymatic analysis using both phosphorous nuclear magnetic resonance spectrometry and colorimetric methods failed to detect the conversion, Thus, the conversion of the pentose shunt intermediates to the D-arabino stereochemistry is not via the expected isomerase but rather must occur via novel metabolic transformations.