Initial reductive reactions in aerobic microbial metabolism of 2,4,6-trinitrotoluene
Because of its high electron deficiency, initial microbial transformations of 2,4,6-trinitrotoluene (TNT) are characterized by reductive rather than oxidation reactions. The reduction of the nitro groups seems to be the dominating mechanism, whereas hydrogenation of the aromatic ring, as described for picric acid, appears to be of minor importance. Thus, two bacterial strains enriched with TNT as a sole source of nitrogen under aerobic conditions, a gram-negative strain called TNT-8 and a gram-positive strain called TNT-32, carried out nitro-group reduction. In contrast, both a picric acid-utilizing Rhodococcus erythropolis strain, HL PM-1, and a 4-nitrotoluene-utilizing Mycobacterium sp. strain, HL 4-NT-1, possessed reductive enzyme systems, which catalyze ring hydrogenation, i.e., the addition of a hydride ion to the the aromatic ring of TNT. The hybride-Meisenheimer complex thus formed (H(-)-TNT) was further converted to a yellow metabolite, which by electrospray mass and nuclear magnetic resonance spectral analyses was established as the protonated dihydride-Meisenheimer complex of TNT (2H(-)-TNT). Formation of hydride complexes could not be identified with the TNT-enriched strains TNT-8 and TNT-32, or with Pseudomonas sp. clone A (2NT(-)), for which such a mechanism has been proposed. Correspondingly, reductive denitration of TNT did not ocurr.