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 (m-DAP), since the activity with L,L-2,6-diaminopimelate is less than 5% of the activity observed with m-DAP. Can use NAD(+) only very poorly since the activity observed in the presence of NAD(+) is about 14% of that with NADP(+). H2O + meso-2,6-diaminoheptanedioate + NADP(+) = (S)-2-amino-6-oxoheptanedioate + H(+) + NADPH + NH4(+) Optimum temperature is 37 degrees Celsius. Amino-acid biosynthesis; L-lysine biosynthesis via DAP pathway; DL-2,6-diaminopimelate from (S)-tetrahydrodipicolinate: step 1/1. Homodimer. Is expressed at a very high level and is reciprocally regulated with dapL and dapF. Belongs to the diaminopimelate dehydrogenase family.