Food Chemistry

1.4 Proteins 75

Table 1.31.Lysinoalanine content of various foods

Food Origin/ Lysinoalanine
Treatment (mg/kg protein)

Frankfurter CPa Raw 0
Cooked 50
Roasted in oven 170
Chicken drums CP Raw 0
Roasted in oven 110
Roasted in
micro wave oven 200
Egg white, fluid CP 0
Egg white Boiled
(3 min) 140
(10 min) 270
(30 min) 370
Baked
(10 min/ 150 ◦C) 350
(30 min/ 150 ◦C) 1100
Dried egg white CP 160–1820b
Condensed milk,
sweetened CP 360–540
Condensed milk,
unsweetened CP 590–860
Milk product for
infants CP 150–640
Infant food CP <55 150
Soya protein isolate CP 0–370
Hydrolyzed
vegetable protein CP 40–500
Cocoa powder CP 130–190
Na-caseinate CP 45–6900
Ca-caseinate CP 250–4320

aCommercial product.
bVariation range for different brand name products.

from animal sources are trypsin, chymotrypsin,
elastase, plasmin and thrombin. Serine pro-
teinases are produced by a great number of
bacteria and fungi, e. g.Bacillus cereus, B. fir-
mus, B. licheniformis, B. megaterium, B. subtilis,
Serratia marcescens, Streptomyces fradiae,
S. griseus, Trititrachium album, Aspergillus
flavus, A. oryzaeandA. sojae.
These enzymes have in common the presence of
a serine and a histidine residue in their active sites
(for mechanism, see 2.4.2.5).
Inactivation of these enzymes is possible
with reagents such as diisopropylfluorophos-
phate (DIFP) or phenylmethanesulfonylfluo-
ride (PMSF). These reagents irreversibly acylate
the serine residue in the active site of the

enzymes:

E−CH 2 OH+FY→E−CH 2 OY+HF

(Y:−PO(iC 3 H 7 O) 2 −SO 2 −CH 2 C 6 H 6 )

(1.158)

Irreversible inhibition can also occur in the

presence of halogenated methyl ketones which

alkylate the active histidine residue (cf. 2.4.1.1),

or as a result of the action of proteinase in-

hibitors, which are also proteins, by interaction

with the enzyme to form inactive complexes.

These natural inhibitors are found in the organs

of animals and plants (pancreas, colostrum,

egg white, potato tuber and seeds of many

legumes; cf. 16.2.3). The specificity of serine