Bifunctional enzyme that catalyzes the first two steps in lysine degradation. H2O + L-saccharopine + NADP(+) = 2-oxoglutarate + H(+) + L-lysine + NADPH H2O + L-saccharopine + NAD(+) = (S)-2-amino-6-oxohexanoate + H(+) + L-glutamate + NADH Amino-acid degradation; L-lysine degradation via saccharopine pathway; glutaryl-CoA from L-lysine: step 1/6. Amino-acid degradation; L-lysine degradation via saccharopine pathway; glutaryl-CoA from L-lysine: step 2/6. Homotetramer. The N-terminal and the C-terminal domains contain respectively the lysine ketoglutarate reductase and saccharopine dehydrogenase activity. In the N-terminal section; belongs to the AlaDH/PNT family. In the C-terminal section; belongs to the saccharopine dehydrogenase family.