Isolation and characterization of enzymes involved in lysine catabolism from sorghum seeds

Nutrition and antioxidant profiling in the unpolished and polished grains of eleven indigenous aromatic rice cultivars

The present study emphasized on the yet-unexplored exhaustive analyses of nutritional and antioxidant parameters in the unpolished and polished grains of eleven indigenous aromatic rice varieties. Tulaipanji appeared to be a highly demanding variety by virtue of having sufficient levels of micronutrients like Fe, Zn and Cu (linked with higher expression of fer2, ZIP and NAS3), inorganic phosphorus, hexose sugars, total amino acids and lysine (correlated with higher expression of glutelin and RLRH1), tocopherol (due to higher HGGT expression), total phenolic content, flavonoids, anthocyanins (concomitant with higher expression of PPO, PAL and ANS), LOX activity and LOX1 gene expression, and overall higher total antioxidant capacity, particularly in the polished grains. The importance of IET-21261, with regard to higher content of phytic acid and total phosphorus (with high IPK1 expression), β-carotene (with high PSY expression) and tocopherol (with high HGGT expression), and of Kalonunia, with respect to cysteine and γ-oryzanol in the polished grains, was also significant. Lower α-amylase enzyme activity and α-amylase expression led to considerable starch accumulation, with lower sucrose content, in the unpolished grains of Radhunipagal and polished grains of Pusa Basmati-1. Paramanya registered the highest content of thiamine and TH1 expression, together with minimum methylglyoxal level (low TPI expression). Paramanya and Radhunipagal maintained a higher pool of majority of the nutritional and antioxidant components in their unpolished grains. The polished grains of all the genotypes showed strikingly lower nutritional constituents, as compared to unpolished grains. The knowledge gained from this study will largely provide a road map to the farmers and rice consumers for making proper choice of the aromatic genotypes for large-scale cultivation and dietary consumption to derive maximum nutritional benefits.

Lysine metabolism in antisense C-hordein barley grains

The grain proteins of barley are deficient in lysine and threonine due to their low concentrations in the major storage protein class, the hordeins, especially in the C-hordein subgroup. Previously produced antisense C-hordein transgenic barley lines have an improved amino acid composition, with increased lysine, methionine and threonine contents. The objective of the study was to investigate the possible changes in the regulation of key enzymes of the aspartate metabolic pathway and the contents of aspartate-derived amino acids in the nontransgenic line (Hordeum vulgare L. cv. Golden Promise) and five antisense C-hordein transgenic barley lines. Considering the amounts of soluble and protein-bound aspartate-derived amino acids together with the analysis of key enzymes of aspartate metabolic pathway, we suggest that the C-hordein suppression did not only alter the metabolism of at least one aspartate-derived amino acid (threonine), but major changes were also detected in the metabolism of lysine and methionine. Modifications in the activities and regulation of aspartate kinase, dihydrodipicolinate synthase and homoserine dehydrogenase were observed in most transgenic lines. Furthermore the activities of lysine α-ketoglutarate reductase and saccharopine dehydrogenase were also altered, although the extent varied among the transgenic lines.

Co-suppression of synthesis of major α-kafirin sub-class together with γ-kafirin-1 and γ-kafirin-2 required for substantially improved protein digestibility in transgenic sorghum

KEY MESSAGE

Co-suppressing major kafirin sub-classes is fundamental to improved protein digestibility and nutritional value of sorghum. The improvement is linked to an irregularly invaginated phenotype of protein bodies.

ABSTRACT

The combined suppression of only two genes, γ kafirin-1 (25 kDa) and γ-kafirin-2 (50 kDa), significantly increases sorghum kafirin in vitro digestibility. Co-suppression of a third gene, α-kafirin A1 (25 kDa), in addition to the two genes increases the digestibility further. The high-digestibility trait has previously only been obtained either through the co-suppression of six kafirin genes (α-A1, 25 kDa; α-B1, 19 kDa; α-B2, 22 kDa; γ-kaf1, 27 kDa; γ-kaf 2, 50 kDa; and δ-kaf 2, 18 kDa) or through random chemical-induced mutations (for example, the high protein digestibility mutant). We present further evidence that suppressing just three of these genes alters kafirin protein cross-linking and protein body microstructure to an irregularly invaginated phenotype. The irregular invaginations are consistent with high pepsin enzyme accessibility and hence high digestibility. The approach we adopted towards increasing sorghum protein digestibility appears to be an effective tool in improving the status of sorghum as a principal supplier of energy and protein in poor communities residing in marginal agro-ecological zones of Africa.

A sorghum xylanase inhibitor-like protein with highly potent antifungal, antitumor and HIV-1 reverse transcriptase inhibitory activities

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The antifungal protein purified in this study is a pH stable and thermostable xylanase inhibitor.

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Sorghum antifungal protein is inhibitory toward various fungal species.

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The sorghum antifungal protein exerts suppressive action on the proliferating hepatoma (HepG2) cells and breast cancer (MCF7) cells.

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Sorghum antifungal protein exerts a highly potent inhibitory activity against HIV-1 reverse transcriptase.

A 25-kDa protein, with an N-terminal amino acid sequence homologous to that of xylanase inhibitor and designated as xylanase inbibitor-like protein (XILP) was purified from sorghum seeds. The isolation protocol consisted of affinity chromatography, ion exchange chromatography, and gel filtration. XILP inhibited mycelial growth in various phytopathogenic fungi. The antifungal activity was thermostable and pH-stable. XILP inhibited proliferation of various cancer cell lines but did not do so in human embryonic liver (WRL 68) cells. There was no mitogenic activity toward mouse splenocytes. XILP reduced the activity of HIV-1 reverse transcriptase with an IC₅₀ of 11.1 μM, but lacked inhibitory activity toward HIV-1 integrase and SARS coronavirus proteinase. In conclusion, sorghum XILP is thermostable and pH stable and exhibits potent antifungal, antiproliferative, and HIV-1 reverse transcriptase inhibitory activities.

Lysine biosynthesis and nitrogen metabolism in quinoa (Chenopodium quinoa): study of enzymes and nitrogen-containing compounds

Aspartate kinase (AK, EC 2.7.2.4), homoserine dehydrogenase (HSDH, EC 1.1.1.3) and dihydrodipicolinate synthase (DHDPS, EC 4.2.1.52) were isolated and partially purified from immature Chenopodium quinoa Willd seeds. Enzyme activities were studied in the presence of the aspartate-derived amino acids lysine, threonine and methionine and also the lysine analogue S-2-aminoethyl-l-cysteine (AEC), at 1 mM and 5 mM. The results confirmed the existence of, at least, two AK isoenzymes, one inhibited by lysine and the other inhibited by threonine, the latter being predominant in quinoa seeds. HSDH activity was also shown to be partially inhibited by threonine, whereas some of the activity was resistant to the inhibitory effect, indicating the presence of two isoenzymes, one resistant and another sensitive to threonine inhibition. Only one DHDPS isoenzyme highly sensitive to lysine inhibition was detected. The results suggest that the high concentration of lysine observed in quinoa seeds is possibly due to a combined effect of increased lysine synthesis and accumulation in the soluble form and/or as protein lysine. Nitrogen assimilation was also investigated and based on nitrate content, nitrate reductase activity, amino acid distribution and ureide content, the leaves were identified as the predominant site of nitrate reduction in this plant species. The amino acid profile analysis in leaves and roots also indicated an important role of soluble glutamine as a nitrogen transporting compound.

The aspartic acid metabolic pathway, an exciting and essential pathway in plants

Aspartate is the common precursor of the essential amino acids lysine, threonine, methionine and isoleucine in higher plants. In addition, aspartate may also be converted to asparagine, in a potentially competing reaction. The latest information on the properties of the enzymes involved in the pathways and the genes that encode them is described. An understanding of the overall regulatory control of the flux through the pathways is undisputedly of great interest, since the nutritive value of all cereal and legume crops is reduced due to low concentrations of at least one of the aspartate-derived amino acids. We have reviewed the recent literature and discussed in this paper possible methods by which the concentrations of the limiting amino acids may be increased in the seeds.

Are high-lysine cereal crops still a challenge?

The essential amino acids lysine and threonine are synthesized in higher plants via a pathway starting with aspartate that also leads to the formation of methionine and isoleucine. Lysine is one of most limiting amino acids in plants consumed by humans and livestock. Recent genetic, molecular, and biochemical evidence suggests that lysine synthesis and catabolism are regulated by complex mechanisms. Early kinetic studies utilizing mutants and transgenic plants that over-accumulate lysine have indicated that the major step for the regulation of lysine biosynthesis is at the enzyme dihydrodipicolinate synthase. Despite this tight regulation, recent strong evidence indicates that lysine catabolism is also subject to control, particularly in cereal seeds. The challenge of producing crops with a high-lysine concentration in the seeds appeared to be in sight a few years ago. However, apart from the quality protein maize lines currently commercially available, the release of high-lysine crops has not yet occurred. We are left with the question, is the production of high-lysine crops still a challenge?