Chemistry and Biology of Proteins - F. Haurowitz 1953

Albumins, globulins, and other soluble proteins
Plant-derived proteins

Plant Proteins are derived mainly from cereal grains and legumes. The seeds intended for Protein Extraction must be thoroughly ground and, if necessary, defatted using diethyl or petroleum ether. Proteins are extracted from the crushed and defatted seed material using aqueous salt solutions, Buffer solutions, or ethyl alcohol. The bulk of the proteins extracted from seeds in this manner belong to globulins, with only 0.1 to 0.5% classified as albumin-type proteins. Globulins soluble in dilute ethyl alcohol are termed prolamins, whereas globulins insoluble in either salt solutions or alcohol, but soluble in dilute acids or alkalis, are known as glutelins [2]. Neither prolamins nor glutelins represent homogeneous proteins; rather, they are mixtures of various structurally similar proteins [163, 164].

Among the globulins soluble in salt solutions, edestin, isolated from hemp seeds, deserves the greatest attention. This same protein group also includes excelsin from Brazil nuts, amandin from almonds, legumin from peas and lentils, phaseolin from Kidney beans, glycinin from soybeans, canavalin and concanavalin from the seeds of Canavalia ensiformis, and globulins from cottonseed, pumpkin, and other plants. All these globulins are extracted from ground seeds using 2–10% sodium chloride solutions. Barium hydroxide is added to the sodium chloride solution to neutralize the organic acids contained in the seeds. Upon dialysis, edestin and certain other globulins precipitate in crystalline form. Edestin can be recrystallized from a 10% sodium chloride solution. Its Amino Acid Composition is given in Table 1. The molecular weights of some globulins are listed below [165, 166]:

Strictly speaking, not all plant-derived globulins can be considered true globulins, as some of them are not precipitated by half-saturation with ammonium sulfate.

Alcohol-soluble seed globulins are called prolamins because they contain high amounts of Proline and amides. It is also evident from Table 1 that they contain very few basic Amino Acids and are exceptionally rich in glutamic acid. For example, gliadin consists of more than 40% glutamic acid. Despite this high glutamic acid content, prolamins do not exhibit an acidic character because the carboxyl groups of all glutamic acid molecules are amidated (see Table 1). Some prolamins completely lack Lysine, which drastically reduces their nutritional value.

Among the prolamins, the most thoroughly studied are gliadin from wheat or rye grains, hordein from barley (Hordeum vulgare), and zein from corn (Zea mays). To obtain these proteins, flour is prepared from the respective grains and then subjected to extraction with 70–80% alcohol. The proteins that pass into the extract are precipitated with diethyl ether [167, 168]. Alternatively, these Proteins can be isolated by kneading dough with Water and extracting it with alcohol after washing away the starch [169]. Prolamins are insoluble in water and absolute alcohol, yet they dissolve readily in 50–80% ethyl alcohol. Zein has also been found to be soluble in certain anionic and cationic detergents [170]. The Molecular Weight of gliadin and hordein is approximately 27 500, while that of zein is 40 000 [171]. However, electrophoretic analysis has revealed that prolamins represent a mixture of two or more components [163]. Gliadin contains two fractions with isoelectric points at pH 5 and 7 [172]1.

Since zein does not contain lysine, it can be assumed that all its free Amino groups are located exclusively in the a-position; careful corresponding studies have shown that zein possesses only one or two free amino groups [173] and, consequently, the zein molecule contains only one or two peptide chains. The zein molecule contains just a single cystine residue [173]. Peptic Hydrolysis of zein yields Peptides terminating in glutamic acid amino groups, whereas tryptic hydrolysis produces peptides containing terminal Alanine residues [173].

Gluten is composed of approximately equal quantities of two proteins: gliadin and glutenin. Glutenin is insoluble in both ethyl alcohol and neutral salt solutions, but it readily dissolves in a 0.2% potassium hydroxide solution and reprecipitates upon neutralization of the alkaline solution with Hydrochloric acid. In all probability, glutenin is a mixture of various proteins and Lipoproteins [163, 164].