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 saccharopine 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 it from developing soybean (Glycine max) seeds. LKR activity of the bifunctional LKR/ SDH possessed relatively high K(m) for its substrates, Lys and alpha-ketoglutarate, suggesting that this activity may serve as a rate-limiting step in Lys catabolism. Despite their linkage, the LKR and SDH enzymes possessed significantly different pH optima, suggesting that SDH activity of the bifunctional enzyme may also be rate-limiting in vivo. We have previously shown that Arabidopsis plants contain both a bifunctional LKR/ SDH and a monofunctional 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 the presence of such a monofunctional SDH enzyme in soybean seeds. These results may provide a plausible regulatory explanation as to why various plant species accumulate different catabolic products of Lys.[1]References
- Purification and characterization of bifunctional lysine-ketoglutarate reductase/saccharopine dehydrogenase from developing soybean seeds. Miron, D., Ben-Yaacov, S., Reches, D., Schupper, A., Galili, G. Plant Physiol. (2000) [Pubmed]
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