Phytate: impact on environment and human nutrition. A challenge for molecular breeding

Crystal structure of Klebsiella sp. ASR1 phytase suggests substrate binding to a preformed active site that meets the requirements of a plant rhizosphere enzyme

The extracellular phytase of the plant-associated Klebsiella sp. ASR1 is a member of the histidine-acid-phosphatase family and acts primarily as a scavenger of phosphate groups locked in the phytic acid molecule. The Klebsiella enzyme is distinguished from the Escherichia coli phytase AppA by its sequence and phytate degradation pathway. The crystal structure of the phytase from Klebsiella sp. ASR1 has been determined to 1.7 A resolution using single-wavelength anomalous-diffraction phasing. Despite low sequence similarity, the overall structure of Klebsiella phytase bears similarity to other histidine-acid phosphatases, such as E. coli phytase, glucose-1-phosphatase and human prostatic-acid phosphatase. The polypeptide chain is organized into an alpha and an alpha/beta domain, and the active site is located in a positively charged cleft between the domains. Three sulfate ions bound to the catalytic pocket of an inactive mutant suggest a unique binding mode for its substrate phytate. Even in the absence of substrate, the Klebsiella phytase is closer in structure to the E. coli phytase AppA in its substrate-bound form than to phytate-free AppA. This is taken to suggest a preformed substrate-binding site in Klebsiella phytase. Differences in habitat and substrate availability thus gave rise to enzymes with different substrate-binding modes, specificities and kinetics.

Choice of foods and ingredients for moderately malnourished children 6 months to 5 years of age

There is consensus on how to treat severe malnutrition, but there is no agreement on the most cost-effective way to treat infants and young children with moderate malnutrition who consume cereal-dominated diets. The aim of this review is to give an overview of the nutritional qualities of relevant foods and ingredients in relation to the nutritional needs of children with moderate malnutrition and to identify research needs. The following general aspects are covered: energy density, macronutrient content and quality, minerals and vitamins, bioactive substances, antinutritional factors, and food processing. The nutritional values of the main food groups--cereals, legumes, pulses, roots, vegetables, fruits, and animal foods--are discussed. The special beneficial qualities of animal-source foods, which contain high levels of minerals important for growth, high-quality protein, and no antinutrients or fibers, are emphasized. In cereal-dominated diets, the plant foods should be processed to reduce the contents of antinutrients and fibers. Provision of a high fat content to increase energy density is emphasized; however, the content of micronutrients should also be increased to maintain nutrient density. The source of fat should be selected to supply optimal amounts of polyunsaturated fatty acids (PUFAs), especially n-3 fatty acids. Among multiple research needs, the following are highlighted: to identify the minimum quantity of animal foods needed to support acceptable child growth and development, to examine the nutritional gains of reducing contents of antinutrients and fibers in cereal- and legume-based diets, and to examine the role of fat quality, especially PUFA content and ratios, in children with moderate malnutrition.

Dietary fibre and phytate--a balancing act: results from three time points in a British birth cohort

An investigation was carried out to determine whether there were significant changes in the intake of dietary fibre (NSP) and phytate of adult men and women in the UK from 1982 (aged 36 years) to 1999 (aged 53 years). The 1253 subjects studied were members of the Medical Research Council National Survey of Health and Development; a longitudinal study of a nationally representative cohort of births in 1946. Food intake was recorded in a 5 d diary at age 36 years in 1982, 43 years in 1989 and 53 years in 1999. The food composition database was amended with revised values for phytate. Outcome measures were mean intakes of total NSP and phytate by year, sex and food source. There were significant changes in total NSP and phytate intake over the three time points. Intakes of NSP rose significantly between 1982 and 1999 for men and women but phytate intakes rose significantly only between 1989 and 1999. Cereal foods were the most important source of both NSP and phytate. Between 1989 and 1999 there was a significant increase in the contribution from pasta, rice and other grains. The present study shows that an increase in dietary fibre that is in accordance with dietary guidelines would almost inevitably be accompanied by a rise in phytate. The increased dietary phytate is discussed in relation to its recognised inhibition of mineral absorption and its merits with regard to protection against some cancers and other diseases of an ageing population.

Rice endosperm iron biofortification by targeted and synergistic action of nicotianamine synthase and ferritin

Nearly one-third of the world's population, mostly women and children, suffer from iron malnutrition and its consequences, such as anaemia or impaired mental development. Iron fortification of food is difficult because soluble iron is either unstable or unpalatable, and non-soluble iron is not bioavailable. Genetic engineering of crop plants to increase iron content has therefore emerged as an alternative for iron biofortification. To date, strategies to increase iron content have relied on single genes, with limited success. Our work focuses on rice as a model plant, because it feeds one-half of the world's population, including the majority of the iron-malnourished population. Using the targeted expression of two transgenes, nicotianamine synthase and ferritin, we increased the iron content of rice endosperm by more than six-fold. Analysis of transgenic rice lines confirmed that, in combination, they provide a synergistic effect on iron uptake and storage. Laser ablation-inductively coupled plasma-mass spectrometry showed that the iron in the endosperm of the transgenic rice lines accumulated in spots, most probably as a consequence of spatially restricted ferritin accumulation. Agronomic evaluation of the high-iron rice lines did not reveal a yield penalty or significant changes in trait characters, except for a tendency to earlier flowering. Overall, we have demonstrated that rice can be engineered with a small number of genes to achieve iron biofortification at a dietary significant level.

Sourdough in gluten-free bread-making: an ancient technology to solve a novel issue?

The increasing demand for high quality gluten-free (GF) bread, clean labels and natural products is raising the need for new approaches in GF bread-making. Sourdough is the foremost fermentation used for baking purposes and it has been proven to be ideal for improving the texture, palatability, aroma, shelf life and nutritional value of wheat and rye breads. These characteristic features derive from the complex metabolic activities of the sourdough-resident lactic acid bacteria and yeasts, e.g. acidification, production of exopolysaccharides, proteolytic- amylolytic- and phytase activity, and production of antimicrobial substances. These effects have been extensively studied and well described for traditional baking, whereas little is known about the role of sourdough in GF baking. Yet, the microbiological and qualitative characterisation of local GF fermented products indicate an overlap with the microbiota of wheat/rye fermentation and suggest that the positive metabolic activities of the sourdough microbiota are still retained during fermentation of GF crops. Thus, the use of sourdough in GF baking may be the new frontier for improving the quality, safety and acceptability of GF bread.

Biofortification of crops with seven mineral elements often lacking in human diets--iron, zinc, copper, calcium, magnesium, selenium and iodine

The diets of over two-thirds of the world's population lack one or more essential mineral elements. This can be remedied through dietary diversification, mineral supplementation, food fortification, or increasing the concentrations and/or bioavailability of mineral elements in produce (biofortification). This article reviews aspects of soil science, plant physiology and genetics underpinning crop biofortification strategies, as well as agronomic and genetic approaches currently taken to biofortify food crops with the mineral elements most commonly lacking in human diets: iron (Fe), zinc (Zn), copper (Cu), calcium (Ca), magnesium (Mg), iodine (I) and selenium (Se). Two complementary approaches have been successfully adopted to increase the concentrations of bioavailable mineral elements in food crops. First, agronomic approaches optimizing the application of mineral fertilizers and/or improving the solubilization and mobilization of mineral elements in the soil have been implemented. Secondly, crops have been developed with: increased abilities to acquire mineral elements and accumulate them in edible tissues; increased concentrations of 'promoter' substances, such as ascorbate, beta-carotene and cysteine-rich polypeptides which stimulate the absorption of essential mineral elements by the gut; and reduced concentrations of 'antinutrients', such as oxalate, polyphenolics or phytate, which interfere with their absorption. These approaches are addressing mineral malnutrition in humans globally.

Speciation and distribution of arsenic and localization of nutrients in rice grains

Arsenic (As) contamination of rice grains and the generally low concentration of micronutrients in rice have been recognized as a major concern for human health. Here, we investigated the speciation and localization of As and the distribution of (micro)nutrients in rice grains because these are key factors controlling bioavailability of nutrients and contaminants. Bulk total and speciation analyses using high-pressure liquid chromatography (HPLC)-inductively coupled plasma mass spectrometry (ICP-MS) and X-ray absorption near-edge spectroscopy (XANES) was complemented by spatially resolved microspectroscopic techniques (micro-XANES, micro-X-ray fluorescence (micro-XRF) and particle induced X-ray emission (PIXE)) to investigate both speciation and distribution of As and localization of nutrients in situ. The distribution of As and micronutrients varied between the various parts of the grains (husk, bran and endosperm) and was characterized by element-specific distribution patterns. The speciation of As in bran and endosperm was dominated by As(III)-thiol complexes. The results indicate that the translocation from the maternal to filial tissues may be a bottleneck for As accumulation in the grain. Strong similarities between the distribution of iron (Fe), manganese (Mn) and phosphorus (P) and between zinc (Zn) and sulphur (S) may be indicative of complexation mechanisms in rice grains.

Do mammals make all their own inositol hexakisphosphate?

A highly specific and sensitive mass assay for inositol hexakisphosphate (InsP₆) was characterized. This centres around phosphorylating InsP₆ with [³²P]ATP using a recombinant InsP₆ kinase from Giardia lambia, followed by HPLC of the ³²P-labelled products with an internal [³H]InsP₇ standard. This assay was used to quantify InsP₆ levels in a variety of biological samples. Concentrations of InsP₆ in rat tissues varied from 10–20 μM (assuming 64% of wet weight of tissue is cytosol water), whereas using the same assumption axenic Dictyostelium discoideum cells contained 352±11 μM InsP₆. HeLa cells were seeded at low density and grown to confluence, at which point they contained InsP₆ levels per mg of protein similar to rat tissues. This amounted to 1.952±0.117 nmol InsP₆ per culture dish, despite the cells being grown in serum shown to contain no detectable (less than 20 pmol per dish) InsP₆. These results demonstrate that mammalian cells synthesize all their own InsP₆. Human blood was analysed, and although the white cell fraction contained InsP₆ at a concentration comparable with other tissues, in serum and platelet-free plasma no InsP₆ was detected (<1 nM InsP₆). Human urine was also examined, and also contained no detectable (<5 nM) InsP₆. These results suggest that dietary studies purporting to measure InsP₆ at micromolar concentrations in human plasma or urine may not have been quantifying this inositol phosphate. Therefore claims that administrating InsP₆ in the diet or applying it topically can produce health benefits by increasing extracellular InsP₆ levels may require reassessment.

Zinc biofortification of cereals: problems and solutions

The goal of biofortification is to develop plants that have an increased content of bioavailable nutrients in their edible parts. Cereals serve as the main staple food for a large proportion of the world population but have the shortcoming, from a nutrition perspective, of being low in zinc and other essential nutrients. Major bottlenecks in plant biofortification appear to be the root-shoot barrier and--in cereals--the process of grain filling. New findings demonstrate that the root-shoot distribution of zinc is controlled mainly by heavy metal transporting P1B-ATPases and the metal tolerance protein (MTP) family. A greater understanding of zinc transport is important to improve crop quality and also to help alleviate accumulation of any toxic metals.

Expression, purification and characterization of a phyA(m)-phyCs fusion phytase

The phyA(m) gene encoding acid phytase and optimized neutral phytase phyCs gene were inserted into expression vector pPIC9K in correct orientation and transformed into Pichia pastoris in order to expand the pH profile of phytase and decrease the cost of production. The fusion phytase phyA(m)-phyCs gene was successfully overexpressed in P. pastoris as an active and extracellular phytase. The yield of total extracellular fusion phytase activity is (25.4+/-0.53) U/ml at the flask scale and (159.1+/-2.92) U/ml for high cell-density fermentation, respectively. Purified fusion phytase exhibits an optimal temperature at 55 degrees C and an optimal pH at 5.5~6.0 and its relative activity remains at a relatively high level of above 70% in the range of pH 2.0 to 7.0. About 51% to 63% of its original activity remains after incubation at 75 degrees C to 95 degrees C for 10 min. Due to heavy glycosylation, the expressed fusion phytase shows a broad and diffuse band in SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis). After deglycosylation by endoglycosidase H (EndoH(f)), the enzyme has an apparent molecular size of 95 kDa. The characterization of the fusion phytase was compared with those of phyCs and phyA(m).