GLUTAMINE-SYNTHETASE OF MYCOBACTERIUM-TUBERCULOSIS - EXTRACELLULAR RELEASE AND CHARACTERIZATION OF ITS ENZYMATIC-ACTIVITY

We have investigated the activity and extracellular release of glutami ne synthetase [L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2] o f Mycobacterium tuberculosis. The purified, homogeneous M. tuberculosi s glutamine synthetase appears to consist of 12 most likely identical subunits of M(r) 58,000, arranged in two superimposed hexagons. In the catalysis of L-glutamine, the enzyme has an apparent K-m for L-glutam ate of approximate to 3 mM at the pH optimum of 7.5. M. tuberculosis r eleases a large proportion (approximate to 30%) of its total measurabl e enzyme activity into the culture medium, a feature that is highly sp ecific for pathogenic mycobacteria. Immunogold electron microscopy rev ealed that M. tuberculosis also releases the enzyme into its phagosome in infected human monocytes. Two potentially important roles for glut amine synthetase in the pathogenesis of M. tuberculosis infection are (i) the synthesis of L-glutamine, a major component of the cell wad of pathogenic but not nonpathogenic mycobacteria, and (ii) the modulatio n of the ammonia level in the M. tuberculosis phagosome, which may in turn influence phagosomal pH and phagosome-lysosome fusion.