Purification and characterization of bifunctional lysine-ketoglutarate reductase/saccharopine dehydrogenase from developing soybean seeds

Both in mammals and plants, excess lysine (Lys) is catabolized via saccharo pine into alpha-amino adipic semialdehyde and glutamate by two consecutive enzymes, Lys-ketoglutarate reductase (LKR) and saccharopine dehydrogenase. (SDH), which are linked on a single bifunctional polypeptide. To study the control of metabolite flux via this bifunctional enzyme, we have purified i t from developing soybean (Glycine max) seeds. LKR activity of the bifuncti onal LKR/SDH possessed relatively high K-m for its substrates, Lys and alph a-ketoglutarate, suggesting that this activity may serve as a rate-limiting step in Lys catabolism. Despite their linkage, the LKR and SDH enzymes pos sessed significantly different pH optima, suggesting that SDH activity of t he bifunctional enzyme may also be rate-limiting in vivo. We have previousl y shown that Arabidopsis plants contain both a bifunctional LKR/SDH and a m onofunctional SDH enzymes (G. Tang, D. Miron, J.X. Zhu-Shimoni, G. Galili [ 1997] Plant Cell 9: 1-13). In the present study, we found no evidence for t he presence of such a monofunctional SDH enzyme in soybean seeds. These res ults may provide a plausible regulatory explanation as to why various plant species accumulate different catabolic products of Lys.