Enzymes of lysine metabolism from Coix lacryma-jobi seeds.

Lugli, Juverlande and Campbell, Adriano and Gaziola, Salete A. and Smith, Richard J. and Lea, Peter J. and Azevedo, Ricardo Antunes (2002) Enzymes of lysine metabolism from Coix lacryma-jobi seeds. Plant Physiology and Biochemistry, 40 (1). pp. 25-32. ISSN 0981-9428

Abstract

Lysine, threonine, methionine and isoleucine are synthesized through the aspartate metabolic pathway. The concentrations of soluble lysine and threonine in cereal seeds are very low. Coix lacryma-jobi (coix) is a maize-related grass and the enzymological aspects of the aspartate metabolic pathway are completely unknown. In order to obtain information on lysine metabolism in this plant species, two enzymes involved in the biosynthesis of these amino acids (aspartate kinase AK, EC 2.7.2.4 and homoserine dehydrogenase HSDH, EC 1.1.1.3) and two enzymes involved in lysine degradation (lysine 2-oxoglutarate reductase LOR, EC 1.5.1.8 and saccharopine dehydrogenase SDH, EC 1.5.1.9) were isolated and partially characterized in coix seeds. AK activity was inhibited by threonine and lysine separately, suggesting the presence of two isoenzymes, one sensitive to lysine and the other sensitive to threonine, with the latter corresponding to approximately 60% of the total AK activity. In contrast to previous results from other plant species, the threonine-sensitive AK eluted from an ion exchange chromatography column at higher KCl concentration than the lysine-sensitive form. The HSDH activity extracted from the seeds was partially inhibited by threonine, indicating the presence of threonine-sensitive and threonine-resistant isoenzymes. LOR and SDH activities were detected only in the endosperm tissue and co-purified on an anion exchange chromatography column, suggesting that the two activities may be linked on a single bifunctional polypeptide, as observed for other plant species. One single SDH activity band was observed on non-denaturing PAGE gels. The Km for saccharopine of SDH was determined as 0.143 mM and the Km for NAD as 0.531 mM. Although SDH activity was shown to be stable, LOR, AK and HSDH were extremely unstable, under all buffer systems tested.