Bifunctional enzyme that catalyzes the first two steps in lysine degradation. The N-terminal and the C-terminal contain lysine-oxoglutarate reductase and saccharopine dehydrogenase activity, respectively. Negatively regulates free Lys accumulation in seeds. H2O + L-saccharopine + NADP(+) = 2-oxoglutarate + H(+) + L-lysine + NADPH H2O + L-saccharopine + NAD(+) = (S)-2-amino-6-oxohexanoate + H(+) + L-glutamate + NADH The LKR activity is stimulated by NaCl. Optimum pH is 7.5 for LKR activity of isoform Long, and 9 for SDH activity of both isoforms Long and Short. 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. Homodimer. Ubiquitous, with higher levels in flowers. Isoform Long is mostly present in young leaves, cotyledons, root tips and mature root parts. Whereas isoform Short is mostly expressed in cotyledons and at low levels in all root parts. In flowers, confined to ovules and vascular tissue of anther filament. In developing and mature seeds, expressed in embryo and the outer layers of the endosperm. Lysine, Sugar starvation, ABA and MeJA induce isoform Long, but not isoform Short (at protein level). Nitrogen starvation repress isoform Long, but not isoform Short (at protein level). Isoform Long and isoform Short are both slightly induced by NaCl and drought stress, but repressed by sugars. Phosphorylation of Ser-458 seems important for the LKR activity. Contains both LKR and SDH activities. Contains only SDH activity. In the N-terminal section; belongs to the AlaDH/PNT family. In the C-terminal section; belongs to the saccharopine dehydrogenase family. Chimeric cDNA. Its C-terminal part is derived from gene At1G61240 whose function is unknown. Originally thought to be an alternative splicing form of At4g33150.