Soluble protein classes
Globulin, the saline-soluble
protein, was the major fraction in silflower seed protein and accounted
for more than half of the total protein (Table 5). Globulin amount is
about 3-fold greater than either water-soluble albumin or alkali-soluble
glutelin. The ethanol-soluble fraction (prolamin) was the least at
8.71%. Silflower globulin content compares well with that of sunflower
meal protein (40-90%), while its albumin content is also within the
range reported for sunflower protein (10-30%) (Gonzalez-Perez and
Vereijken, 2007). Glutelin in sunflower is only a minor fraction
(Gonzalez-Perez and Vereijken, 2007) but such is not the case for
silflower glutelin, which, at 19%, was the second highest content after
the globulins. Sunflower globulin and albumin are reported to have
glutamic acid-glutamine, glycine, aspartic acid-asparagine, and arginine
as the most abundant amino acids (Baudet and Mosse, 1977), but the
albumin is also rich in lysine and sulfur-containing amino acids
(cysteine + methionine) (Kortt and Caldwell, 1990; Egorov et al., 1996),
which globulin lacks. The prolamin fraction generally has glutamine and
proline in the greatest amounts, but is deficient in lysine and
tryptophan (Shewry, Napier, and Tatham, 1995). Glutelin peptide
sequences were reported to be predominantly glutamine, proline and
glycine (high molecular weight) or serine, glutamine, proline, and
phenylalanine (low molecular weight) (Wieser, Seilmeier, and Belitz,
1988). The amounts of soluble protein classes in silflower seed do not
conform to those found in cereals (28% albumins + globulins, 40%
glutelins, 33% prolamins) or high-protein seeds (92% albumins +
globulins, 7% glutelins, <1% prolamins) (Nikokyris and
Kandylis, 1997).