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. Exhibits a high substrate specificity for meso-2,6-diaminopimelate, since L,L-2,6-diaminopimelate, D,D-2,6-diaminopimelate, L-glutamate, L-alanine, L-leucine, L-valine, L-aspartate, L-threonine, L-homoserine, L-methionine, L-lysine, L-serine, L-phenylalanine, L-tyrosine, L-tryptophan, L-ornithine, L-histidine, L-arginine, D-glutamate, and D-alanine are not substrates for the oxidative deamination reaction. Can use NAD(+) only poorly since the activity observed in the presence of NAD(+) is about 3% of that with NADP(+). H2O + meso-2,6-diaminoheptanedioate + NADP(+) = (S)-2-amino-6-oxoheptanedioate + H(+) + NADPH + NH4(+) L,L-2,6-diaminopimelate and D,D-2,6-diaminopimelate competitively inhibit the oxidative deamination of meso-2,6-diaminopimelate. The enzyme is also inhibited by L-cysteine, and by p-chloromercuribenzoate, iodoacetic acid and HgCl(2) in vitro. Optimum pH is about 9.8 for the oxidative deamination of meso-diaminopimelate and 7.9 for the reductive amination of L-2-amino-6-oxopimelate. Amino-acid biosynthesis; L-lysine biosynthesis via DAP pathway; DL-2,6-diaminopimelate from (S)-tetrahydrodipicolinate: step 1/1. Homodimer. Is composed of three domains: a dinucleotide binding domain, a dimerization domain, and a substrate-binding domain. Belongs to the diaminopimelate dehydrogenase family.