578 Part V: Fruits, Vegetables, and Cereals
with inhibition activity against these enzymes. Pre-
sumably, these enzyme inhibitors contribute to plant
defense mechanisms and/or possibly intervene in
the complex regulation of plant metabolic processes.
Starch-Degrading Enzymes and Their Inhibitors
-amylases or, in full, 1,4--D-glucan glucanohy-
drolases (E.C. 3.2.1.1) catalyze the hydrolysis of the
internal 1,4--D-glucan linkages in starch compo-
nents.
Rye synthesizes two groups of -amylases: low
pI (pI 5.5) and high pI (pI 5.8). The low-pI -
amylases are synthesized during grain development,
particularly in the pericarp (Dedio et al. 1975). Their
activity decreases during the later stages of grain
development and is low at maturity. The high-pI
-amylases are synthesized during germination. In
germinated rye, the high-pI groups represent a
high proportion of the total amylase activity
(MacGregor et al. 1988). Gabor et al. (1991) iso-
lated a high-pI -amylase from germinated rye,
also called “germination-specific”-amylase, which
showed optimal activity at pH 5.5 and is stable at pH
6.0–10.0 after storage at 4°C. The temperature opti-
mum over a 3 minute incubation period was 65°C,
but 40% of the activity was lost when the enzyme
was incubated without substrate for 4 hours at 55°C.
Täufel et al. (1991) also isolated a germination-
specific -amylase from rye and reported a pH opti-
mum of 5.0, pH stability between pH 5.0 and 7.0
and thermal stability up to 55°C.
Variation in -amylase levels exists between dif-
ferent rye lines (Masojc and Larssonraznikiewicz
1991a). Masojc and Larssonraznikiewicz (1991b)
showed the existence of low and high -amylase
genotypes. In contrast, Hansen et al. (2004) stated
that the -amylase activity is mostly influenced by
harvest year, attributing only a small effect to geno-
type.
-amylase inhibitors can reduce the activity of
one or more -amylases and are mainly found in
two major families, that is, the “cereal trypsin/-
amylase inhibitor” and the “Kunitz” or “-amylase/
subtilisin inhibitor” families (Garcia-Olmedo et al.
1987). The cereal trypsin/-amylase inhibitor fami-
ly represents the major part of the albumin and glob-
ulin fraction of the endosperm and consists of
monofunctional inhibitors, active against either
trypsin or exogenous -amylase. The Kunitz-type
inhibitors are bifunctional inhibitors of about 20 k,
containing two intramolecular disulphide bridges.
They can simultaneously inhibit both subtilisin and
endogenous -amylases, in particular the germina-
tion specific, high-pI -amylase.
Täufel et al. (1991, 1997) purified two proteina-
ceous inhibitors of -amylase from rye: a regulation
or R-type inhibitor, which only inhibits the endoge-
nous germination-specific -amylase, and a defense
or D-type inhibitor, which only inhibits exogenous
-amylases of animal and human origin. The in-
hibitors are present in different tissues of the rye
kernel (Täufel et al. 1997). The R-type inhibitor pre-
dominantly occurs in the bran, whereas the D-type
inhibitor is enriched in the germ and the endosperm.
The high D-type inhibitor activity in these kernel
sections is consistent with its assumed defense func-
tion, as it can protect the starch against exogenous
-amylases from insects and rodents. The levels of
both types of inhibitor vary with growing conditions
(Täufel et al. 1997). Rye varieties cultivated under
drought conditions show high D-type inhibitor ac-
tivities and low R-type inhibitor activities, whereas
the opposite is true when they are cultivated under
wet conditions. Small variations in -amylase in-
hibitor activities exist between sprout-stable and
sprout-sensitive rye genotypes (Täufel et al. 1997).
Whereas -amylase activity increases rapidly after
24 hours of germination, the R- and D-type inhibitor
activities are stable during the 72 hours of germina-
tion (Täufel et al. 1997). The -amylase inhibitors
obviously have no importance at the early stages of
growth.Arabinoxylan-Degrading Enzymes and Their In-
hibitors Arabinoxylan structural and physico-
chemical properties can be impacted by enzymes
such as arabinofuranosidases, xylosidases, esterases,
and endoxylanases, with the latter being by far the
most relevant in cereal processing.
Endoxylanases or, in full, endo-1,4--D-xylan
xylanohydrolases(E.C. 3.2.1.8) hydrolyze internal
1,4 linkages between -D-xylopyranose residues
of (arabino-) xylan molecules, generating (arabino-)
xylan fragments with lower molecular weight and
(un)substituted xylooligosaccharides. Their action
can thus lead to (partial) solubilization of water-
unextractable arabinoxylans and to a decrease in
molecular weight of water-extractable and/or solubi-
lized arabinoxylans. The susceptibility of arabinox-
ylans to endoxylanase attack probably depends on
their substitution degree and their substitution pat-
tern as well as on their linkages to other cell wall