Degradation of lysine in rice seeds: Effect of calcium, ionic strength, S-adenosylmethionine and S-2-aminoethyl-L-cysteine on the lysine 2-oxoglutarate reductase-saccharopine dehydrogenase bifunctional enzyme

Lysine biosynthesis has been extensively studied and the regulatory enzymes characterized in some of the most important crop plants, however, much les s is known about the lysine degradation pathway. Lysine 2-oxoglutarate redu ctase (LOR) and saccharopine dehydrogenase (SDN) have recently been partial ly purified and characterized from plants, and have been shown to exist as a single bifunctional polypeptide, We have further characterized these enzy mes from rice endosperm in relation to Ca2+ and ionic strength modulation, Optimum pH values of 7.0 and 8.0 were obtained for LOR and SDH, respectivel y, The LOR domain of the polypeptide was modulated by Ca2+ and ionic streng th, whereas the SDH domain was not, It would appear that the modulation by Ca2+ and ionic strength of LOR is a common feature among plant LOR enzymes, S-adenosylmethionine (SAM) did not produce any significant effect on eithe r enzyme activity, indicating that it only plays a role in the regulation o f lysine biosynthesis. The effect of S-2-aminoethyl-L-cysteine (AEC) as bot h a substrate and an inhibitor of LOR activity was also tested, AEC was sho wn to partially substitute for lysine as a substrate for LOR, but was also able to inhibit LOR activity, possibly competing with lysine at the active site. The higher K-m for AEC compared to lysine may reflect a lower binding affinity for AEC.