Food Chemistry

15.2 Individual Constituents 699

Table 15.23.Substrate specificity of glutathione dehydrogenase (GSH-DH) from wheat

Substrate Relative activity (%) Kinetic constants

VM(nkat/ml) KM(mmol/l)

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-Acceptorc
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.

aConcentration ofL-threo-DHA: 0.5mol/l.
bReaction system:L-threo-DHA 0.5 mmol/l, Cys-Gly 34 mmol/l.
cReaction system:H-acceptor 0.29 mmol/l, GSH 0.5 mmol/l.
dStructures of the corresponding ascorbic acid diastereomers, cf. 15.4.1.4.1.
eConcentration of GSH: 3 mmol/l.

n.a.: not analyzed.

Table 15.24.Occurrence of low-molecular thiols in
wheat floura

Thiol Concentration
(nmol/gflour)

Glutathione (GSH) 100
Glu-Cys 17
Cys-Gly 5
Cysteine 13

aOrigin: DNS (ash 0.78%).

Table 15.25.Activity of ascorbic acid oxidase (AO) in
wheat flour

Wheat cultivar AOa

Domino 60
Otane 40
Norseman 39
Amethyst 30
Sapphire 21
Brock 18

aActivity at pH 6.2(25◦C): nmolL-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 protein-

bound 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 predom-

inantly localized in the germ and in the aleu-

rone layer. Therefore its concentration in flour

increases as the extraction grade increases (Ta-

ble 15.27).

During dough making, GSH reacts very quickly

undergoing disulfide interchange with the flour

proteins PSSP:

GSH+PSSPPSSG+PSH (15.4)

If high-molecular gluten proteins are cleaved, the

viscosity of the dough drops. Rheological meas-