Diaphorase catalyzed biotransformation of RDX via N-denitration mechanism

Previously, we hypothesized that hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) can be biotransformed by anaerobic sludge via three different routes: (1) direct ring cleavage via alpha-hydroxylation of a-CH(2) group, (2) reduction of one of the -NO(2) groups to -NO, (3) N-denitration prior to ring cleavage. The present study describes biotransformation of RDX via route 3 by a diaphorase (EC 1.8.1.4) from Clostridium kluyveri using NADH as electron donor. The removal of RDX was accompanied by the formation and accumulation of nitrite ion (NO(2)(-)), formaldehyde (HCHO), ammonium (NH(4)(+)), and nitrous oxide (N(2)O). None of the RDX-nitroso products were detected. The ring cleavage product methylenedinitramine was detected as the transient intermediate. Product stoichiometry showed that each reacted RDX molecule produced one nitrite ion and the product distribution gave a carbon (C) and nitrogen (N) mass balance of 91 and 92%, respectively, supporting the occurrence of a mono-denitration step prior to the ring cleavage and decomposition. Severe oxygen mediated inhibition (92% inhibition) of RDX biotransformation and superoxide dismutase-sensitive cytochrome c reduction indicated the potential involvement of an anion radical RDX(.-) prior to denitration. A comparative study between native- and apo-enzymes showed the possible involvement of flavin mononucleotide (FMN) in catalyzing the transfer of a redox equivalent (e/H(+)) from NADH to RDX to produce RDX(.-) responsible for secondary decomposition.