Enzymes of lysine metabolism from Coix lacryma-jobi seeds |
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| Affiliation: | 1. First Affiliated Hospital of Shihezi University, School of Medicine, Xinjiang, China;2. Key Laboratory of Xinjiang Endemic and Ethnic Diseases, Shihezi University, School of Medicine, Xinjiang, China;3. Office for Education to International Students, School of Medicine, Shihezi University, Xinjiang, China;4. Department of Cardiovascular Internal Medicine, First Affiliated Hospital of Xinjiang University, School of Medicine, Xinjiang, China;5. Department of Orthopedics, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China;1. School of Pharmacy, Yantai University, Yantai, Shandong 264005, PR China;2. School of Life Science, Yantai University, Yantai, Shandong 264005, PR China;3. State key laboratory of Long-acting and Targeting Drug Delivery Technologies (luye Pharma Group Ltd.), Yantai, Shandong 264003, PR China;1. Center for Clinical and Translational Science, Mayo Clinic, Rochester, MN, United States;2. University of Puerto Rico Medical Sciences Campus, School of Medicine, San Juan, Puerto Rico;3. Department of Orthopedic Surgery, Mayo Clinic, Rochester, MN, United States;4. Department of Physiology and Biomedical Engineering, Mayo Clinic, Rochester, MN, United States;5. Biomedical Imaging Resource, Mayo Clinic, Rochester, MN, United States;6. Division of Endocrinology, Metabolism and Nutrition, Department of Internal Medicine, Mayo Clinic, Rochester, MN, United States |
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| 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. |
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