The Enzymology of Lysine Catabolism in Rice Seeds — Isolation, Characterization, and Regulatory Properties of a Lysine 2‐Oxoglutarate Reductase/Saccharopine Dehydrogenase Bifunctional Polypeptide

In plant, the catabolism of lysine has only been studied in some detail in maize. The enzymes lysine α‐oxoglutarate reductase (also known as lysine α‐ketoglutarate reductase; LOR) and saccharopine dehydrogenase (SDH), which convert lysine into saccharopine, and saccharopine into glutamic acid and 2–aminoadipate 6‐semialdehyde, respectively, were isolated from immature rice seeds and partially purified through a three‐step purification procedure involving ammonium sulphate precipitation, and anion‐exchange and gel‐filtration chromatographies, leading to a final yield of 30% for LOR and 24% for SDH. The molecular masses estimated by gel‐filtration chromatography on a Sephacryl S200 column and by native non‐denaturing PAGE using Ferguson plots were 203 kDa for both enzymes by gel‐filtration and 202 kDa for both enzymes by native non‐denaturing PAGE. A second band of LOR and SDH activities on native gels was observed for both enzymes with an estimated molecular mass of 396 kDa, which indicated a multimeric structure. Kinetic studies were consistent with an ordered sequence mechanism for LOR, where 2‐oxoglutarate is the first substrate and saccharopine is the last product. The results observed for the LOR/SDH activity ratios during purification, the copurification in all three steps, the molecular masses, the relative mobilities on native non‐denaturing gels and the pl estimated for LOR and SDH suggest the existence of a bifunctional polypeptide containing LOR and SDH activities.