2009_Book_FoodChemistry

15.2 Individual Constituents 699
Table 15.23. Substrate specificity of glutathione dehydrogenase (GSH-DH) from wheat
Substrate Relative activity (%) Kinetic constants
V M (nkat/ml)
H-Donor
Glutathione (GSH) 100 362 a 1.8
Cysteine 0 0 n.a.
Cysteinyl glycine (Cys-Gly) 0 1.3 b n.a.
γ-Glutamyl cysteine (Glu-Cys) n.a. 37 a 5.5
H-Acceptor c
Dehydroascorbic acid (DHAsc) d
L-threo 100 275 e 0.14
L-erythro 67 n.a. n.a.
D-erythro 60 305 1.2
D-threo 16 n.a. n.a.
a Concentration of L-threo-DHA: 0.5mol/l.
b Reaction system: L-threo-DHA 0.5 mmol/l, Cys-Gly 34 mmol/l.
c Reaction system: H-acceptor 0.29 mmol/l, GSH 0.5 mmol/l.
d Structures of the corresponding ascorbic acid diastereomers, cf. 15.4.1.4.1.
e Concentration of GSH: 3 mmol/l.
n.a.: not analyzed.
K M (mmol/l)
Table 15.24. Occurrence of low-molecular thiols in
wheat flour a
Thiol
Glutathione (GSH) 100
Glu-Cys 17
Cys-Gly 5
Cysteine 13
a Origin: DNS (ash 0.78%).
Concentration
(nmol/gflour)
Table 15.25. Activity of ascorbic acid oxidase (AO) in
wheat flour
Wheat cultivar AO a
Domino 60
Otane 40
Norseman 39
Amethyst 30
Sapphire 21
Brock 18
a Activity at pH 6.2(25 ◦ C): nmol L-threo-ascorbic acid
per minute and g of flour.
substrates p-nitrophenyl-β-D-xylopyranoside and
p-nitrophenyl-α-L-arabinofuranoside. A water
soluble arabinoxylan gave arabinose and xylose
on incubation as the main products and xylobiose
and xylotetraose as the side products. The results
show that low activities of arabinofuranosidase,
xylosidase and endo-xylanase are found in wheat
flour.
15.2.3 Other Nitrogen Compounds
Wheat contains glutathione and cysteine in the
free state as thiol compounds (GSH, CSH), in the
oxidized forms (GSSG, CSSC) and in the proteinbound
forms (GSSP and CSSP) (Table 15.26).
Reduction of GSSP and CSSP releases GSH and
CSH respectively, e. g., with dithioerythritol.
It has been shown that glutathione is predominantly
localized in the germ and in the aleurone
layer. Therefore its concentration in flour
increases as the extraction grade increases (Table
15.27).
During dough making, GSH reacts very quickly
undergoing disulfide interchange with the flour
proteins PSSP:
GSH + PSSP ⇋ PSSG + PSH (15.4)
If high-molecular gluten proteins are cleaved, the
viscosity of the dough drops. Rheological meas-