Food Biochemistry and Food Processing

576 Part V: Fruits, Vegetables, and Cereals

consistent with a higher proportion of repetitive
sequences in the former group (Shewry et al. 1982).
The two -secalin types differ in aggregation
behavior. The 40k -secalins predominantly occur
as monomers containing intramolecular disulphide
bonds, while the 75k -secalins are present predom-
inantly as disulphide-linked aggregates (Field et al.
1983, Gellrich et al. 2003, Shewry et al. 1983). A
possible explanation for this different aggregation
behavior is the presence or absence of cysteine res-
idues in positions that are favorable or unfavorable
for formation of intermolecular disulphide bonds.
Alternatively, it might result from the specific action
of enzymes responsible for the formation of such
bonds (Field et al. 1983, Shewry et al. 1983). Gel-
lrich et al. (2001, 2004b) showed that the eight cys-
teine residues of the C-terminal domain of 75k -
secalin are linked by intramolecular disulphide
bonds and that the cysteine residue located in posi-
tion 12 of the N-terminal domain of the 75k -
secalin forms an intermolecular disulphide bond

with 75k -secalins. The cysteine residue in the

N-terminal domain is unique to the 75k -secalins

and is likely to be responsible for their aggregative

nature (Gellrich et al. 2003, 2004b).

The 40k -secalins are homologous to the wheat

-gliadins and to the B hordeins of barley (Field et

al. 1983; Gellrich et al. 2001, 2003; Shewry et al.

1982), whereas the 75k -secalins differ from them

due to the presence of additional repetitive se-

quences rich in glutamine and proline and their

occurrence as aggregates. The latter are suggested to

be homologous to the LMW glutenin subunits

(LMW-GS) of wheat (Shewry et al. 1982) (Fig.

25.1).

The S-poor or -secalins are quantitatively minor

components, accounting for about 19% of the total

secalin fraction (Gellrich et al. 2003). They have

SDS-PAGE molecular weights of about 48 k to 53 k

(Field et al. 1983, Gellrich et al. 2003, Shewry et

al. 1983). The amino acid compositions of the -

secalins are characterized by high levels of

Figure 25.1.Schematic representation of the classification of rye and wheat proteins.