Prolamins

M.p. 207°C. The naturally occurring substance is dextrorotatory. Arginine is one of the essential amino-acids and one of the most widely distributed products of protein hydrolysis. It is obtained in particularly high concentration from proteins belonging to the prolamine and histone classes. It plays an important role in the production of urea as an excretory product. 

Prolamines. Proteins insoluble in water, but dissolving in aqueous alcohol solutions. Found in the seeds of cereals. 

Cereal proteins when classified by the Osborne sequential extraction method yield four different classes albumins, which are water soluble, globulins, which are soluble in salt solutions, prolamins, which are soluble in alcohol-water mixtures, and glutelins, which are soluble in dilute acid or alkali. Chen and Bushuk added a fifth fraction by dividing the glutelin into two fractions, one soluble in dilute (0.05 m) acetic acid and the other insoluble in this reagent.5... 

Biochemical and genetic researchers divide the proteins into three groups S-poor, i.e. sulfur poor, S-rich, i.e. sulfur rich, and HMW-prolamins, i.e. high molecular weight prolamins. 

These organelles occur in the endosperm of cereal grains and their structures are tissue specific. They are about 2-5 im in diameter and often contain globoid and occasionally crystalloid inclusions. Prolamin accumulates in small or large spherical bodies. Crystalline protein bodies are the sites of accumulation of nonprolamin storage proteins. 

Proteins historically have been classified on the basis of their solubility in water (albumins) salt solution (globulins) alcohol (prolamines) and alkali (glutelins) (20). 

Only proteins that contain proline bind polyphenols. Asano et al. (1982) demonstrated that the haze-forming activity of a protein is roughly proportional to the mole percentage of proline it contains (see Fig. 2.3). DNA has codes for exactly 20 amino acids. If each of these were equally present in a protein, there would be 5 mol% of each one. In fact, most proteins have much less proline than this. There are a few exceptions. Casein has about 8 mol% proline and the grain prolamins (proline-rich, alcohol-soluble proteins) are even higher. Hordein, the barley prolamin, contains about 20 mol% proline. As a result, it readily forms haze with polyphenols and is the main beer haze-active (HA) protein. Hordein contains even more glutamine (Q) than proline (P), and often these amino acids are adjacent in the protein (see Fig. 2.4). In fact, the sequence P-Q-Q-P occurs... 

A response surface model of the effects of HA protein concentration (gliadin, the wheat prolamin), HA polyphenol concentration (tannic acid, TA), alcohol, and pH on the amount of haze formed was constructed using a buffer model system (Siebert et al., 1996a). Figure 2.12 shows the effects of protein and polyphenol on haze predicted by the model at fixed levels of pH and alcohol. The model indicates that as protein increases at fixed polyphenol levels, the haze rises to a point and then starts to decline. Similarly, when polyphenol increases at a fixed protein level, the haze increases to a maximum and then declines. 

Wheat, rye, and barley have a common ancestral origin in the grass family. Oats are more distantly related to the analogous proteins in wheat, rye, and barley and the oat prolamins (avenin) have substantially lower proline content. Avenin accounts for 5-15% of the total protein in oats, whereas in wheat, barley, and rye, prolamins constitute 40-50% of the total protein (Kilmartin et al., 2006). Some investigators believe that there are similarities between the protein structure of oats and some wheat-like sequences, which may indicate that large amounts of oats could potentially be toxic to patients with celiac disease. However, the putative toxic amino acid sequences are less frequent in avenin than in other prolamins, which explains the less toxic nature of oats (Arentz-Hansen et al., 2004 Ellis and Ciclitira, 2001, 2008 Shan et al., 2005 Vader et al., 2002, 2003). 

Cornell, H. J., McLachlan, A., and Cullis, P. G. (2002). Extraction of cereal prolamins and their toxicity in coeliac disease. ]. Biochem. Mol. Biol. Biophys. 6,151-158. 

Most of the applications of HPLC for protein analysis deal with the storage proteins in cereals (wheat, corn, rice, oat, barley) and beans (pea, soybeans). HPLC has proved useful for cultivar identihcation, protein separation, and characterization to detect adulterations (illegal addition of common wheat flour to durum wheat flour) [107]. Recently Losso et al. [146] have reported a rapid method for rice prolamin separation by perfusion chromatography on a RP POROS RH/2 column (UV detection at 230nm), sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE), and molecular size determination by MALDl-MS. DuPont et al. [147] used a combination of RP-HPLC and SDS-PAGE to determine the composition of wheat flour proteins previously fractionated by sequential extraction. 

Prolamin in rice grains. Kinki Daigaku Nogakubu Kiyo 1992 25 51-54. 

A significantly higher amoimt of protein is found in beans from South Africa compared to beans from Botswana. The amount of protein in beans from Namibia is between the contents from the two other countries (Holse et al., 2010). The variation in protein content might be due to different concentration of nitrogen in the soils. Bower et al. (1988) found that globulins are the most abimdant (53%) protein constituents in morama beans. The beans furthermore consist of albumins (23.3%), prolamins (15.5%), alkali soluble glutelins (7.7%), and acid-soluble glutelins (0.5%). 

Identification of oat (Avena) cultivars by HPLC was first reported by Lookhart and coworkers (153-155) in combined electrophoresis/HPLC experiments. The HPLC technique used was a modification of the procedure described by Bietz (137) for wheat. Generally, the prolamin fraction, i.e., the alcohol-soluble fraction, of oat species generates complex polyacrylamide gel-electrophoresis (PAGE) and RP-HPLC patterns, with increasing complexity as ploidy of the selections increased. Readily (visible) identification of the cultivars was possible only when PAGE and RP-HPLC results were combined. An HPLC procedure for the characterization of the major oat protein fractions was developed by Lapvetelainen et al. (156). Salt-soluble, alcohol-soluble, and alkali-soluble protein fractions were extracted with 0.1 M NaCl, 52% ethanol, and 1% SDS in 0.05 M borate (pH 10), respectively. For the five cultivars examined, RP-HPLC separations of salt- and alkali-soluble proteins were very similar, whereas the prolamin fraction enabled culti-var differentiation, except for very closely related cultivars. 

FG Chirdo, CA Fossati, MC Anon. Fractionation of wheat, barley, and rye prolamines by cation exchange FPLC. J Agric Food Chem 42 2460 -2465, 1994. 

GL Lookhart, LD Albers. Correlations between reversed-phase high-performance liquid chromatography and acid- and sodium dodecyl sulfate-polyacrylamide gel electrophoretic data on prolamines from wheat sister lines differing widely in baking quality. Cereal Chem 65 222-227, 1988. 

GL Lookhart, Y Pomeranz. Characterization of oat species by polyacrylamide gel electrophoresis and high performance liquid chromatography of their prolamin proteins. Cereal Chem 62 162-166, 1985. 

GL Lookhart. Identification of oat cultivars by fingerprinting their prolamin fractions via polyacrylamide gel electrophoresis (PAGE) and reversed phase-high performance liquid chromatography (RP-HPLC). Cereal Foods World 29 507, 1984. 

TABLE 6.3 Amino acid composition of Eleusine flour and prolamin... 

FM3 and FM6 are prolamin fractions. Prolamins were extracted from FM flour with 50% (v/v) aqueous propan-l-ol, 2% (v/v) acetic acid, and 2% (v/v) 2-mercaptoethanol, reduced and alkylated prior to separation on RP HPLC. 

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