Catalyzes the conversion of 4-hydroxy-tetrahydrodipicolinate (HTPA) to tetrahydrodipicolinate (Probable). Uses NADPH as a reductant with much more efficiency than NADH. (S)-2,3,4,5-tetrahydrodipicolinate + H2O + NAD(+) = (2S,4S)-4-hydroxy-2,3,4,5-tetrahydrodipicolinate + H(+) + NADH (S)-2,3,4,5-tetrahydrodipicolinate + H2O + NADP(+) = (2S,4S)-4-hydroxy-2,3,4,5-tetrahydrodipicolinate + H(+) + NADPH Is inhibited by high concentrations of NADH. Is stable for up to 48 hours at 80 degrees Celsius. Has a thermal denaturation midpoint (Tm) of 95.7 degrees Celsius. Amino-acid biosynthesis; L-lysine biosynthesis via DAP pathway; (S)-tetrahydrodipicolinate from L-aspartate: step 4/4. Homotetramer. Belongs to the DapB family. Was originally thought to be a dihydrodipicolinate reductase (DHDPR), catalyzing the conversion of dihydrodipicolinate to tetrahydrodipicolinate. However, it was shown in E.coli that the substrate of the enzymatic reaction is not dihydrodipicolinate (DHDP) but in fact (2S,4S)-4-hydroxy-2,3,4,5-tetrahydrodipicolinic acid (HTPA), the product released by the DapA-catalyzed reaction.