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.
Miron
D
D
Department of Plant Sciences, The Weizmann Institute of Science, Rehovot, Israel.
Ben-Yaacov
S
S
Reches
D
D
Schupper
A
A
Galili
G
G
D000595:Amino Acid Sequence; D004591:Electrophoresis, Polyacrylamide Gel; D007700:Kinetics; D008969:Molecular Sequence Data; D012443:Saccharopine Dehydrogenases / Q000737:chemistry / Q000302:isolation & purification* / Q000378:metabolism; D012639:Seeds / Q000201:enzymology*; D013025:Glycine max / Q000196:embryology*; D013045:Species Specificity
D012443:Saccharopine Dehydrogenases