A Proteomic Approach to Identify Zein Proteins upon Eco-Friendly Ultrasound-Based Extraction

Zein is a type of prolamin storage protein that has a variety of biomedical and industrial applications. Due to the considerable genetic variability and polyploidity of the starting material, as well as the extraction methods used, the characterization of the protein composition of zein requires a combination of different analytical processes. Therefore, we combined modern analytical methods such as mass spectrometry (MS), Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), atomic force microscopy (AFM), or Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) for a better characterization of the extracted zein. In this study, we present an enhanced eco-friendly extraction method, including grinding and sieving corn seeds, for prolamins proteins using an ultrasonic extraction methodology. The use of an ultrasonic homogenizer, 65% ethanol extraction buffer, and 710 µm maize granulation yielded the highest protein extraction from all experimental conditions we employed. An SDS PAGE analysis of the extracted zein protein mainly revealed two intense bands of approximatively 20 and 23 kDa, suggesting that the extracted zein was mostly α-zein monomer. Additionally, MS analysis revealed as a main component the α-zein PMS2 (Uniprot accession no. P24450) type protein in the maize flour extract. Moreover, AFM studies show that extracting zein with a 65% ethanol and a 710 µm granulation yields a homogeneous content that could allow these proteins to be employed in future medical applications. This research leads to a better understanding of zeins content critical for developing new applications of zein in food and pharmaceutical industries, such as biocompatible medical vehicles based on polyplexes complex nanoparticles of zein with antimicrobial or drug delivery properties.

From 'green' technologies to 'red' antioxidant compounds extraction of purple corn: a combined ultrasound-ultrafiltration-purification approach

BACKGROUND

A pilot scale process consisting of ultrasound-assisted extraction, ammonium sulfate precipitation, cross-flow ultrafiltration and AB-8 macroporous resins purification aiming to recover anthocyanins and zein from purple corn (PC) was optimized and scaled-up. The effects of five independent variables (ethanol concentration, liquid to solid ratio, ultrasound temperature, time and power) were discussed and the most influential factors were optimized.

RESULTS

The highest total anthocyanin (0.45 ± 0.01 g kg^-1 ) and zein (17.14 ± 1.73 g kg^-1 ) contents from purple corn were obtained using an ultrasound power of 105 W, an extraction time of 90 min, an ethanol concentration of 74% and a liquid to solid ratio of 26:1, at 70 °C, and this was consistent with the predicted values (0.46 and 17.36 g kg^-1 , for anthocyanin and zein, respectively). Subsequently, ammonium sulfate precipitation was used to isolate anthocyanins and zein. After cross-flow ultrafiltration, zein (6.30 g) was obtained with 80% purity. Anthocyanins were purified by AB-8 macroporous resins, resulting in 1.60 g of anthocyanins. High-performance liquid chromatography-electrospray ionization-mass spectrometry analysis revealed eight different anthocyanins in purple corn extracts.

CONCLUSION

From the results obtained in the present study, it can be concluded that the proposed extraction-separation-filtration-purification method applied under the optimal conditions could be scaled-up to recover anthocyanins and zein simultaneously. Moreover, under the selected conditions, no significant degradation of anthocyanins was observed. © 2018 Society of Chemical Industry.

Ultrasound-based protein determination in maize seeds

The need for a simple and accurate method for protein estimation in alcoholic extracts led to the reexamination of the optimum conditions of a colorimetric assay based on the biuret reaction. Sonication time and the other experimental parameters were optimized after kinetics study on the extraction of either zein or total proteins. Zein extraction and purity were investigated by (1)H and (13)C NMR spectroscopy, SDS-PAGE electrophoresis, and UV-visible spectrophotometry (UV-vis). A zein assay was proposed, which involves the reaction of copper ions in copper phosphate powder with zein extracted in ethanolic solutions under strong alkaline environment. Furthermore, we extended this procedure to determine total proteins in maize samples simultaneously with their ultrasonic-assisted (US) extraction with an alkaline-alcoholic solution. Proteins in both types of extracts were well characterized by UV-vis spectroscopy. However, the 545 nm absorbance of the violet-colored supernatants which is proportional to the protein content was found to be the key parameter of the improved biuret-based protein assay. Comparison of values obtained by this procedure and by Micro-Kjeldahl method was in excellent agreement. A scaled-down procedure agreed well with the standard procedure. Enhanced accuracy and repeatability was found in protein determination in maize using the modified biuret method. The optimization of reagent concentrations and incubation times were studied as well.

Recovery and characterization of α-zein from corn fermentation coproducts

Zeins were isolated from corn ethanol coproduct distiller's dried grains (DDG) and fractionated into α- and β γ-rich fractions. The effects of the ethanol production process, such as fermentation type, protease addition, and DDG drying temperature on zein recovery, were evaluated. Yield, purity, and molecular properties of recovered zein fractions were determined and compared with zein isolated from corn gluten meal (CGM). Around 29-34% of the total zein was recovered from DDG, whereas 83% of total zein was recovered from CGM. Process variations of cooked and raw starch hydrolysis and fermentation did not affect the recovery, purity, and molecular profile of the isolated zeins; however, zein isolated from DDG of raw starch fermentation showed superior solubility and film forming characteristics to those from conventional 2-stage cooked fermentation DDG. Protease addition during fermentation also did not affect the zein yield or molecular profile. The high drying temperature of DDG decreased the purity of isolated zein. SDS-PAGE indicated that all the isolated α-zein fractions contained α-zein of high purity (92%) and trace amounts of β and γ-zeins cross-contamination. Circular dichroism (CD) spectra confirmed notable changes in the secondary structure of α-zeins of DDG produced from cooked and raw starch fermentation; however, all the α-zeins isolated from DDG and CGM showed a remarkably high order of α-helix structure. Compared to the α-zein of CGM, the α-zein of DDG showed lower recovery and purity but retained its solubility, structure, and film forming characteristics, indicating the potential of producing functional zein from a low-value coproduct for uses as industrial biobased product.

An acidic method of zein extraction from DDGS

Zein with a higher intrinsic viscosity and phosphorus content, similar protein content, lower yellowness, and at potentially much lower cost than commercially available zein was obtained from distillers dried grains with solubles (DDGS). A novel extraction method using acidic conditions in the presence of a reducing agent has been used to obtain about 10% aqueous ethanol soluble zein from DDGS. The optimum pH, time, temperature, and amount of reducing agent that can produce zein with high quality and yield have been developed. In addition to the zein, about 17% oil based on the dry weight of DDGS has also been obtained during zein extraction. The zein obtained from this research is expected to be suitable for use as fibers, films, and binders and in paints.

Fractionation of zein by size exclusion chromatography

Zein is a group of alcohol-soluble corn proteins, which consists of several individual proteins. A single-step gel filtration chromatography method was developed to fractionate individual zeins from ethanol extracts of whole corn. A Superdex prep 75 column was used with different mobile phases to fractionate the zeins, which were analyzed by SDS-PAGE and UV spectrophotometry. With 70% aqueous ethanol as the mobile phase, fractions containing a mixture of alpha-zein/beta-zein and alpha-zein/delta-zein were obtained. With ammonium bicarbonate added to the 70% ethanol mobile phase, it was possible to obtain beta-zein and delta-zein fractions devoid of other proteins. However, all fractions containing alpha-zein also contained minor amounts of delta-zein and/or beta-zein. Almost all fractions also contained non-protein impurities.

Novel Prefractionation Method Can Be Used in Proteomic Analysis

For the first time, a novel prefractionation method used in proteomic analysis was developed, which is performed by a novel aqueous two-phase system (NATPS) composed of n-butanol, (NH(4))(2)SO(4), and water. It can separate proteomic proteins into multigroups by one-step extraction. The phase-separation conditions of n-butanol solutions were studied in the presence of commonly used inorganic salts. The NATPS was subsequently developed. Using human serum albumin, zein, and gamma-globulin as model proteins, the separation effectiveness of the NATPS for protein was studied under affection factors, i.e., pH, n-butanol volume, protein, or salt concentration. The model and actual protein samples were separated by the NATPS and then directly used for gel electrophoresis without separating the target proteins from phase-forming reagents. It revealed that the NATPS could separate proteomic proteins into multigroups by one-step extraction. The NATPS has the advantages of rapidity, simplicity, low cost, biocompability, and high efficiency. It need not separate target proteins from the phase-forming reagents. The NATPS has great significance in separation and extraction of proteomic proteins, as well as in methodology.

Characterization of a 19 kDa alpha-zein of high purity

A highly pure alpha-zein was extracted from corn flour using ethanol (95%). Subsequently, ion-exchange chromatography was performed, using SP-Sepharose that yielded a highly homogeneous protein. This protein migrated as a single band in 20% SDS-PAGE and in pH gradient gels, showing an isoelectric point of 6.8. Mass spectrometry (MALDI-TOF-MS) showed a single peak with a molecular mass of 24 535 Da. It was identified as Z19, when comparing the sequence obtained in an automatic Edman sequencer with the Swissprot database using BLAST. The molar extinction coefficient, determined by dry weight in 70% methanol, was 12 415.49 M(-1) cm(-1) at 280 nm. Light scattering showed its presence in a monodispersed state of 44-66 kDa aggregates in methanol (70%). Circular dichroism spectra allowed the estimation of an alpha-helix content that was lower than the one found for a mixture of two alpha-zeins but with a higher content of beta sheets.

Cytoskeletal proteins are coordinately increased in maize genotypes with high levels of eEF1A

The opaque2 (o2) mutation increases the Lys content of maize (Zea mays) endosperm by reducing the synthesis of zein storage proteins and increasing the accumulation of other types of cellular proteins. Elongation factor 1A (eEF1A) is one of these proteins, and its concentration is highly correlated with the amount of other Lys-containing proteins in the endosperm. We investigated the basis for this relationship by comparing patterns of protein accumulation and gene expression between a high (Oh51Ao2) and a low (Oh545o2) eEF1A inbred, as well as between high and low eEF1A recombinant inbred lines obtained from their cross. The content of alpha-zein and several cytoskeletal proteins was measured in high and low eEF1A inbred lines, and the levels of these proteins were found to correlate with that of eEF1A. To extend this analysis, we used an endosperm expressed sequence tag microarray to examine steady-state levels of RNA transcripts in developing endosperm of these genotypes. We identified about 120 genes coordinately regulated in association with eEF1A content. These genes encode proteins involved in several biological structures and processes, including the actin cytoskeleton, the endoplasmic reticulum, and the protein synthesis apparatus. Thus, higher levels of eEF1A in o2 mutants may be related to a more extensive cytoskeletal network surrounding the rough endoplasmic reticulum and increased synthesis of cytoskeleton-associated proteins, all of which contribute significantly to the Lys content of the endosperm.

Comparative efficiencies of isopropyl and tert-butyl alcohols for extracting zeins from maize endosperm

Protein of endosperm of maize grains originating from three wild-type inbreds and their opaque-2 versions were solubilized in diverse extracts (E) by the sequential use of 0.5 M NaCl, water (E(1,2)), alcohol plus a reducing agent (E(3)), and salt plus a reducing agent (E(4)). Zeins were isolated in extracts E(3) and E(4) obtained by using 55% (w/w) isopropyl alcohol (i-PrOH) + 0.2% dithiothreitol (DTT) followed by 0.5 M NaCl + 0.2% DTT buffered at pH 10 or 60% tert-butyl alcohol (t-BuOH) + 0.2% DTT followed by 0.5% sodium acetate + 0.2% DTT in 30% t-BuOH. For a given genotype the percentage of extracted zeins was independent of the nature of the alcohol. The latter had a slight effect on the respective magnitude of E(3) and E(4): E(3) increased at the expense of E(4) when t-BuOH was substituted to i-PrOH for their isolation. The percentage of the total endosperm nitrogen present in E(3) + E(4) was identical to that of fractions F(II) + F(III) + F(IV) isolated according to the classical Landry-Moureaux extraction procedure. SDS-PAGE analysis revealed the presence of all types of zeins (alpha, beta, gamma, and delta) in E(3) and F(III), residual zeins in E(4) isolated with t-BuOH, and streaking only in E(4) and F(IV) isolated with NaCl at pH 10. The data together with those of the literature were discussed with regard to the influence of procedure on the yield of zeins using alcoholic extraction.

Extraction and solubility characteristics of zein proteins from dry-milled corn

Zein isolation by aqueous ethanol extraction from dry-milled corn produces a mixture of zeins, covalently linked polymers (dimers, tetramers, etc.) and higher-molecular-weight aggregates, some of which were not soluble in aqueous alcohol. The insoluble particles were identified as protein aggregates which form when the extraction solution is heated, particularly under alkaline conditions. The insoluble protein aggregates were not present in zein isolated by the same method from corn gluten meal. Zeins extracted from corn gluten meal and dry-milled corn were fractionated (by differential solubility) to identify differences in their polypeptide compositions. Using polyacrylamide gel electrophoresis, beta- and gamma-zeins were detected in dry-milled corn, but only trace amounts of beta-zein were found in corn gluten meal. Treatment of dry-milled corn with 0.55% lactic acid and 0.2% sulfur dioxide at 50 degrees C for 6 h before ethanol extraction resulted in a 50% increase in zein isolate yield with high solubility (98%). This pre-extraction treatment cleaved disulfide linkages of the beta- and gamma-zeins and significantly reduced insoluble aggregates in zein isolates.

Isolation and quantitation of zeins in waxy and amylose-extender and wild flint and dent maize endosperm using a new solvent sequence for protein extraction

Protein distribution in endosperm of maize grains differing by their texture, flint or dent, and by their genotype, wild or waxy or amylose-extender, was examined by the successive use of 0.5 M NaCl, 0.5 M NaCl plus 0.6% 2-mercaptoethanol (2ME) at neutral and then alkaline pH, and 55% 2-propanol plus 0.6% 2ME as extractants. Proteins extracted in the presence of 2ME were characterized by their size polymorphism and amino acid composition. Proteins isolated with NaCl plus 2ME at neutral pH corresponded with a mixture of gamma-zein (27 kDa) and glutelin-like proteins. Proteins isolated with NaCl plus 2ME at pH 10 were a mixture of gamma-zeins (27 and 16 kDa) and beta-zeins (14 kDa). Alcohol-soluble proteins consisted of alpha-, beta-, and delta-zeins, alpha subunits being predominant. Zein quantitation was improved by weighing the nitrogen percentage of extracts by their zein content, as estimated from the data on amino acid composition. The data reported by Wolf et al. (Cereal Chem. 1975, 52, 765) were integrated to the results of this work to suggest the occurrence of an inverse correlation between amylose in starch and zeins in proteins.

Salt-steered partitioning of proteins in polymeric two-phase systems based on N,N-dimethylformamide

The polymeric two-phase (liquid liquid) system, based on N,N-dimethylformamide (DMFA) and the two polymers Ficoll and poly(ethylene glycol). has been investigated. The partitioning of the water-insoluble protein fraction, zein (from corn), was affected by the addition of various salts that were soluble in DMFA. The effect of the salts has been correlated to their partition coefficients. The data have been fitted to a theory for partitioning of polyelectrolytes in aqueous two-phase systems. The two-phase system has been used for partial fractionation of zein by counter-current distribution.

Expression of maize gamma zein C-terminus in Escherichia coli

Prolamins containing a highly conserved cysteine-rich C-terminal domain have been poorly expressed as soluble protein in model systems such as Escherichia coli. Possible reasons have included a combination of the reducing environment of the bacterial cytoplasm and protein secondary structure. Using a bacterial thioredoxin fusion expression system, full-length native gamma zein, native gamma zein C-terminus, and modified gamma zein C-terminus, containing 13 amino acid changes, were found to accumulate up to 58, 50, and 42% of the total cellular protein, respectively. The native gamma zein C-terminus fusion protein was six times more soluble (70%) than the full-length fusion protein (12%), four times more soluble than the N-terminus (19%), and eight times more soluble than the modified C-terminus (9%). The modified C-terminal domain contained amino acid changes that improved the lysine, isoleucine, and tryptophan content, while removing two evolutionarily conserved cysteines and one nonconserved cysteine. Expression of the native C-terminal domain without thioredoxin resulted in decreased solubility (13%) and decreased expression (8%). In contrast, coexpression with thioredoxin resulted in a sevenfold increase in solubility (86%). These results suggest that insolubility of full-length gamma zein results from structural interactions of the N-terminus and that solubility of the C-terminal domain is dependent on proper disulfide bond formation. The ability to express the C-terminal domain of gamma zein as soluble protein should allow future identification of important structural elements in gamma zein and similar proteins.

Maternal-specific demethylation and expression of specific alleles of zein genes in the endosperm of Zea mays L

Zeins constitute 60-70% of maize endosperm protein. Zein genes are specifically transcribed in the endosperm, and a correlation has been established between tissue-specific expression and demethylation. Three inbred lines and their reciprocal crosses were analysed to assess for allele-specific differences in methylation, transcription and translation. DNAs from endosperm, embryo and seedling tissues analysed by cleavage with methylation-sensitive restriction enzymes and Southern blot hybridization with zein cDNA and genomic sequences show that specific demethylation of zein sequences occurs only in endosperm and is restricted to the maternal complements. Steady-state transcript accumulation of zein mRNA assessed by RNase protection assay reveals qualitative and quantitative differences among endosperm RNAs of the inbreds and of their reciprocal hybrids. Moreover, two-dimensional gel electrophoresis of zein proteins identified polypeptides that are maternally imprinted in reciprocal crosses. These results indicate that endosperm-specific expression of specific zein alleles may occur via parental imprinting and disclose a possible role of methylation in regulating the expression of genes differently contributed in the endosperm by the maternal and paternal genomes.

opaque-15, a maize mutation with properties of a defective opaque-2 modifier

An opaque mutation was identified that reduces gamma-zein synthesis in maize endosperm. The mutation, opaque-15, causes a 2- to 3-fold reduction in gamma-zein mRNA and protein synthesis and reduces the proportion of the 27-kDa gamma-zein A gene transcript. Although the protein bodies in opaque-15 are similar in size and morphology compared to wild type, there are fewer of them in developing endosperm cells. The opaque-15 mutation maps near the telomere of chromosome 7L, coincident with an opaque-2 modifier locus. Based on its phenotype, opaque-15 appears to be a mutation of an opaque-2 modifier gene.

Poly(ethylene glycol)methacrylate-acrylamide copolymer media for hydrophobic protein and low temperature electrophoresis

The properties of poly(ethylene glycol)methacrylate-acrylamide copolymer media (PEGMACs) were exploited in two ways: (i) in the first-dimensional gel of two-dimensional electrophoresis (2-DE) of hydrophobic proteins and (ii) for high speed, high-resolution electrophoresis at low temperatures. In the first application, improved resolution and yield of isoelectric focusing (IEF) separations for the hydrophobic protein zein was achieved compared to IEF in standard polyacrylamide gels. This appears promising as a candidate approach for higher resolution 2-DE mapping of uncharacterized hydrophobic proteins. In the second application, PEGMACs compatible with hydroorganic antifreeze buffer systems below cooling of the gels to low temperatures (-20 degrees C), which allowed greater current to be tolerated during electrophoresis. PEGMAC gels enabled us to perform sixfold faster electrophoretic separations and achieve threefold improved resolution of six standard proteins at the lower temperatures in a direct comparison with the normal sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) system. If this approach is coupled with more precise instrumentation to control low temperatures during electrophoresis, greater separation speeds and resolution may be anticipated.

[Extraction and characterization of zeins from kernels of 10 maize cultivars]

The study consisted in the extraction and characterization of zeins of Venezuelan maizes, according to their solubility and molecular properties. The cultivars analyzed were the hybrids Ceniap PB8, Tocoron 127, Arichuna, Obregón, Ceniap 3, Corocito 101 and the varieties Máquina del Ceniap, Venezuela-1 and Venezuela-1 Opaco-2. Zeins were extracted with 70% ethanol and fractionated by two methods: 95% ethanol and column filtration with Sephadex G-200. The molecular weight was determined by electrophoresis in a discontinuous polyacrilamide gel with sodium dodecyl sulfate (PAGE-SDS). The results demonstrated that zeins account for 36.57% of the total protein present in normal corn grain and 9.38% in Opaque-2 corn. Prolamines presented a soluble fraction in 95% ethanol (alpha zeins) which represented 33,12% of zeins and another insoluble (beta zeins) the 66.88%. In column fractionation, three fractions were obtained, two major A and C and another B in minor quantity, which varied in proportion between cultivars. Zeins which were reduced with 2-mercaptoethanol separated into two subunits with molecular weights of 21,300 and 24,300 daltons, whereas unreduced zeins presented large sized aggregates which remained at the origin and large numbers of bands (nine) whose molecular weights oscillated between 24,300 and 87,000 daltons. The electrophoretic patterns of normal maizes were similar, but differ from the pattern of the Opaque-2. In this maize, zeins reduced presented only the band with molecular weight 24,300 daltons and the unreduced zeins showed those with molecular weights of 87,000; 78,500; 48,500 and 24,300 daltons. Reduced alpha and beta are made up of the same basic components of zeins. Likewise unreduced alpha zeins are made up of the same polypeptides as unreduced zeins. The difference between Opaque-2 zeins and normal ones in that they contain more alpha zeins and less polypeptides of high molecular weight.

The maize regulatory locus Opaque-2 encodes a DNA-binding protein which activates the transcription of the b-32 gene

The maize locus, Opaque-2, controls the expression in developing endosperm of structural genes encoding a family of storage proteins, the 22 kd zeins, and an abundant albumin, termed b-32. It is shown that the promoter of the b-32 gene is activated in vivo in the presence of the O2 gene product and that the information necessary for this activation resides in a 440 bp DNA fragment containing five O2 binding sites (GATGAPyPuTGPu). Two of these sites are embedded in copies of the 'endosperm box', a motif thought to be involved in endosperm-specific expression, which is also represented in 22 kd zein promoters. The O2 protein is also shown to be capable of binding in vitro and activating in vivo, its own promoter.

An immunodominant site of gamma-zein1 is in the region of tandem hexapeptide repeats

The immunochemical data from studies with polyclonal antisera to gamma-zein1, the 27 kD component of the maize prolamin, indicated that the region containing 8 tandem repeats of the sequence PPPVHL is an immunodominant site. In one case, the entire antibody repertoire of an antiserum recognized epitope(s) within this region. Three 17-mer oligopeptides corresponding to the predicted antigenic epitopes of gamma-zein1 were synthesized and reacted with three different anti-gamma-zein1 sera in order to map antigenic sites in the intact protein. These antisera yielded positive reactions with a 17-mer peptide (peptide 37), which was not in a hydrophilic maximum but derived from the repeat region. The same antisera gave little or no reaction with other peptides (peptides 38 and 39), both of which were in a hydrophilic maximum. In addition, an antiserum to peptide 37 reacted strongly with both the homologous antigen and the intact gamma-zein1. Peptide 37 also blocked the binding of antisera to gamma-zein1 in competition assays. Subsequently, the shorter 6-mer (peptide 82) and 12-mer (peptide 80) versions of peptide 37 were synthesized, and both reacted with anti-peptide 37 serum and also with each of the three anti-gamma-zein1 sera. In these reactions and in competition assays, the reactivity and the blocking ability increased in proportion to the length of the peptide. Based on these data, it was concluded that the repeat region of gamma-zein1 is the site of one or more continuous immunodominant epitopes. The data also suggest that the repeat region is exposed on the surface of the folded protein and probably occur as a mobile, random coil.

Quantitative analysis of the accumulation of Zein mRNA during maize endosperm development

In order to characterize the heterogeneity and expression of maize zein genes, we constructed and characterized a cDNA library of endosperm mRNAs. Clones from the library that were of sufficient size to be full-length or near full-length copies of zein mRNA were characterized by restriction enzyme mapping and cross-hybridization analysis. Based on these comparisons we found three classes of zein sequences corresponding to proteins of Mr 22,000, five corresponding to proteins of Mr 19,000, and a single one corresponding to a protein of Mr 15,000. Representative clones from these nine groups were used as probes to measure levels of the corresponding mRNAs in developing endosperms. It was found that these groups represent varying amounts of transcripts that range from 2 to 20% of the total endosperm mRNA population. For the Mr 19,000 and Mr 22,000 zein clones there is a correlation between the amount of mRNAs and the apparent number of genes in the genome. The relative level of mRNA for the Mr 15,000 zein was found to be 3 times that of the Mr 22,000 and Mr 19,000 zeins, suggesting that these genes are transcribed at a higher rate during endosperm development or that their mRNAs are more stable.

Zein synthesis in the embryo and endosperm of maize mutants

Zein synthesis in the developing (22 and 50 days postpollination) endosperm and embryo of maize (Zea mays L.) double mutants, brittle-1; opaque-2 and brittle-2;opaque-2, were compared and correlated with sucrose concentration and RNase activity in order to test the hypothesis that high sucrose concentrations may prevent the interaction between zein polyribosomes and endoplasmic reticulum and make the zein mRNAs more susceptible to hydrolysis by high RNase activity, resulting in a severe reduction in zein synthesis. The double-mutant combinations of opaque-2 with each of the starch-deficient mutants, brittle-1 and brittle-2, maintained not only a high sucrose concentration in the endosperm but also a higher RNase activity than either one of the single mutants alone. Consequently, these double mutants severely suppressed the synthesis of two major zein components in their endosperms. In contrast to the endosperm system, embryos of the double mutants produced amounts of zein (and electrophoretic patterns) similar to that of the opaque-2 embryo, and their embryos contained levels of sucrose and RNase activity comparable to that of the o2 and normal control. These results are consistent with the notion that a posttranscriptional degradation of zein mRNAs by RNase, rather than a specific transcriptional block, is involved in the endosperm to suppress zein synthesis in these double mutants.

Zein of maize grain: II--The charge heterogeneity of free subunits

The subunits present as monomers in unreduced zein and isolated as fraction M by gel filtration, were chromatographed on sulfoethyl-cellulose. Three major subfractions were detected and characterized. Each of them, submitted to electrophoresis at pH 3.5, migrated as a single band corresponding to each of the three major electrophoretic forms seen in fraction M at the same pH. The presence of lysine in some polypeptides, suggested by amino acid composition data, was confirmed by electrophoretic analysis of carbamylated subfractions at pH 3.5. At pH 8.9 each subfractions was further resolved into three cationic bands in starch gel and three (or more) anionic bands in polyacrylamide gel. The same fractionation was also obtained by submitting the major electroforms of fraction M, as isolated at pH 3.5, to isoelectric focusing. Based on these observations, the most probable distributions of basic amino acids in subunits detected by electrophoresis at pH 8.9 were specified and compared to those recently published for several zein clones. The presence per polypeptide chain of three carboxyl groups and occasionally of one lysine would be a feature of zein originating from maize hybrid Inra 260.

Zein of maize grain: I--Isolation by gel filtration and characterization of monomeric and dimeric species

Unreduced zein chromatographed on Sephadex G 200 in 8 M urea, on G 100 in 1.5 or 2.5% sodium dodecyl sulfate (SDS) and on hydroxypropylated G 100 in 70% ethanol was resolved into two minor fractions A and B and two major ones D and M irrespective of the medium. The quantitative importance of the fraction M was dependent on the isolation conditions of zein. It decreased from 53% of the proteins contained in ethanolic extract and chromatographed as they were extracted, to 40% of the purified zein. The molecular weight values obtained from SDS-polyacrylamide gel electrophoresis and amino acid compositional data indicated that fractions D and M, as isolated from purified zein in the presence of ethanol, represented respectively dimeric and monomeric forms of a mixture of Mr 22 000 and 24 000 polypeptides with threonine or phenylalanine as NH2-terminal residue. Electrophoretic analysis of selectively carbamylated fraction M on starch gel at pH 3.5 revealed that zein subunits comprised several polypeptides differing in the number and the nature of basic amino acids. At least one of these polypeptides contained one lysyl residue.

Separation of cereal proteins by reversed-phase high-performance liquid chromatography

Cereal proteins have been extremely difficult to purify and characterize owing to their heterogeneity, poor solubility and tendency to polymerize. High-performance liquid chromatography (HPLC) on a 300 A reversed-phase (RP)(C18) support (Syn Chropak RP-P), using acetonitrile as organic modifier in the presence of trifluoroacetic acid, has been found to be capable of high-resolution separations of these proteins; the resolution is often better than that obtained by any other chromatographic or electrophoretic method. Examples are presented showing separations of low-molecular-weight gliadins, omega-gliadins and ethanol-soluble reduced glutenin subunits from wheat and of zein from corn. In addition, proteins may be directly extracted from ground single kernels and subsequently analyzed by RP-HPLC; applications in genetic studies, in breeding programs and in varietal identification are proposed. In addition to its high resolution, RP-HPLC is superior to most other methods in speed, sensitivity, reproducibility and suitability for quantitation. Polypeptide chains of molecular weight up to 133,000 are recovered in high yields and the column capacity is high, demonstrating that RP-HPLC is suitable for both preparative and analytical separations of proteins. RP-HPLC resolves proteins primarily on the basis of differences in surface hydrophobicity, so it therefore complements, rather than duplicates, other techniques that separate proteins on the basis of size or charge. RP-HPLC promises to become an invaluable technique for the fractionation and characterization of proteins from cereals and other sources.

[Zein of maize grain. Preparation and characterization]

A laboratory procedure for isolation and purification of zein from grains of 4 varieties of Maize was described. The preparations were characterized by their physicochemical properties. Upon polyacrylamide gel electrophoresis in sodium dodecyl sulphate (SDS), native zein (from INRA 260 hybrid) was resolved into 2 major classes with average molecular weights of 45,000 and 22,000. After reduction with mercaptoethanol zein contained only two subunits of 22,000 and 24,000 daltons. Upon starch gel electrophoresis in 6 M urea at pH 3.5, native zein exhibited five major or medium intensity bands and several minor ones. The latter, under reducing conditions, disappeared to reinforce the major bands or to yield some new minor bands. Amino acid analysis revealed a very low content of lysine. The NH2-terminal amino acids were determined to be threonine and phenylalanine with a preponderance of the former. Zeins isolated from the varieties studied appeared tohave the same NH2-terminal residues and similar amino acid compositions with an arginine/histidine ratio ranging from 1.1 to 1.2. They differed in relative importance of components, detected by electrophoresis in the presence of SDS or urea. Changes in zein characteristics with the grain genotype allow one to conclude that the components of molecular weights of 22,000 and 24,000 consist of several subunits differing in charge and amino acid content.

[Electrophoretic and chromatographic studies of zein]

Total zein is isolated from fat-free maize flour and studied using electrophoresis in acrylamide gel gradient (7-30%) and gel filtration through Sephacryl S-200 (Superfine) and Sepharose CL-6B. Molecular weights of electrophoretic and chromatographic fractions are determined. 12 electrophoretic and 5-7 chromatographic fractions are found in zein, their molecular weight being 40000-280000 daltons.

Fractionation of water-insoluble protein using sephacryl S-200 in formamide

The ability of Sephacryl S-200 to fractionate water-insoluble protein by gel permeation chromatography was investigated. Elution of protein standards from colums of Sephacryl S-200 equilibrated with formamide, an excellent solvent for both hydrophobic and hydrophilic proteins, was molecular weight dependent. Advantages of chromatography of water-insoluble proteins using Sephacryl S-200 include rapidity, recovery of protein free of detergent or solvent, safety, ability to purify large amounts of protein, and ability to separate proteins as large as 100,000 daltons. Separation of water-insoluble polypeptides in a crude preparation of gliadin in formamide demonstrated the practicality of the method.