Involved in norspermidine biosynthesis. Catalyzes the synthesis of carboxynorspermidine from L-aspartate 4-semialdehyde and 1,3-diaminopropane. Is also slightly active with putrescine as a substrate. carboxynorspermidine + H2O + NADP(+) = H(+) + L-aspartate 4-semialdehyde + NADPH + propane-1,3-diamine carboxyspermidine + H2O + NADP(+) = H(+) + L-aspartate 4-semialdehyde + NADPH + putrescine Activated by dithiothreitol and inhibited by SH-reactive compounds. Optimum pH is 7.25-7.5. Optimum temperature is 37 degrees Celsius. Homodimer. Belongs to the saccharopine dehydrogenase family. carboxynorspermidine synthase subfamily.