Barley for Food and Health: Science, Technology, and Products

70 BARLEY: GENETICS AND NUTRIENT COMPOSITION
Storage proteins include the major alcohol-soluble prolamins, along with glutelins
and minor amounts of globulins. Prolamins, called hordeins in barley, are
the major storage proteins in the endosperm, accounting for 35 to 50% of the total
nitrogen (Kirkman et al. 1982). Osborne coined the name prolamins for this distinct
group of proteins, to reflect the high content of proline, a nonessential amino
acid and amide nitrogen. Barley hordein is a complex mixture of polypeptides
that has been divided on the basis of amino acid composition into three groups:
sulfur rich (B + γ), sulfur poor (C), and high-molecular-weight prolamins (D).
Amino acid analyses of these groups revealed a composition of 30 mol % or
more of the nonessential amino acids glutamic acid and glutamine, and less than
1 mol % of the essential amino acid lysine. Glutelins were initially thought to
be a distinct group of proteins; however, it has been determined that many of
the component proteins of glutelins are structurally related to prolamins (Shewry
and Tatham 1990). Although not soluble in aqueous alcohol, these components
may be considered as prolamins since the individual reduced subunit peptides
are alcohol soluble and rich in prolamine and glutamine. Glutelins contain less
lysine than albumins and globulins, and they are somewhat higher in lysine than
true hordein (prolamin) in barley (Shewry et al. 1984; Shewry and Tatham 1987).
The nonstorage proteins, albumins and globulins, are structural components
of cell walls and metabolic proteins such as enzymes. Some of the nonstorage
proteins may accumulate in large enough quantities as to have a secondary storage
role. Nonstorage proteins are found primarily in the aleurone and embryo,
accounting for 15 to 30% of the total grain nitrogen. Lysine comprises 5 to 7%
of the albumin–globulin fraction, compared to less than 1% in hordeins of most
barley, and is intermediate between these two levels in glutelins. Threonine is also
found in greater concentration in the albumins and globulins. Thus, as a result
of increased lysine and threonine, the nonstorage proteins are more nutritionally
balanced than the storage proteins (Munck 1972; Brandt 1976; El-Negoumy
et al. 1977; Kirkman et al. 1982; Shewry et al. 1984).
Protein and amino acid levels in barley are influenced strongly by both genetic
and environmental growing conditions, although the latter has the greater effect.
Available soil nitrogen is a major environmental factor influencing protein levels
and amino acid levels. Even though total amino acid levels are increased with
elevated protein synthesis in the presence of ample soil nitrogen and moisture,
they are not increased proportionally to one another. Rather, individual amino
acids that are produced are representative of the composition of storage proteins
(hordeins), which are increased selectively by soil nitrogen. Therefore, high protein
levels in barley increased by nitrogen fertilizer generally mean high levels
of the nonessential glutamic acid, proline, and glutamine and lower levels of the
essential amino acids, particularly lysine and threonine. The negative correlation
between protein and lysine levels in most barley is due to amino acid composition
differences in the four major protein fractions and the overwhelming prevalence
of low-lysine hordein in most high-protein barley.
Genetic control of protein level in barley is quantitatively inherited and is
considered to be extremely complex. Only limited genetic information about total