Catalyzes the reversible NADPH-dependent reductive amination of L-2-amino-6-oxopimelate, the acyclic form of L-tetrahydrodipicolinate, to generate the meso compound, D,L-2,6-diaminopimelate. Probably plays a role in lysine biosynthesis. Is highly specific for meso-2,6-diaminopimelate as the electron donor, since the following amino acids are inert for the oxidative deamination reaction: DL-2-aminopimelate, D-glutamate, L-glutamate, D-aspartate, L-aspartate, D-alanine, L-alanine, D-valine, L-valine, D-lysine, L-lysine, D-phenylalanine, L-phenylalanine, D-leucine, L-leucine, D-threonine, L-threonine, D-serine, L-serine, D-tryptophan, L-tryptophan, D-cysteine, L-cysteine, D-histidine, L-histidine, D-methionine, D-arginine, D-proline, D-asparagine, D-glutamine, D-isoleucine and D-ornithine. Moreover, exclusively uses NADP as the electron acceptor for the oxidative deamination of meso-DAP; NAD is inert. H2O + meso-2,6-diaminoheptanedioate + NADP(+) = (S)-2-amino-6-oxoheptanedioate + H(+) + NADPH + NH4(+) The enzyme is completely inhibited by p-chloromercuribenzoate and HgCl(2) in vitro. Thioglycollate, L-cysteine and Cu(2+) also strongly inhibit the enzyme. Optimum pH is about 10.5. No activity is lost after 30 minutes incubation at pHs between 5.0 and 11.0. Optimum temperature is 65 degrees Celsius. Thermostable. No activity is lost after 30 minutes incubation at temperatures below 60 degrees Celsius, while half of the activity is lost after 30 minutes incubation at 65 degrees Celsius. Amino-acid biosynthesis; L-lysine biosynthesis via DAP pathway; DL-2,6-diaminopimelate from (S)-tetrahydrodipicolinate: step 1/1. Homodimer. Belongs to the diaminopimelate dehydrogenase family.