Food Chemistry

1.4 Proteins 71

for example, lactose or glucose, which yield
protein-bound ε-N-deoxylactulosyl-1-lysine or
ε-N-deoxyfructosyl-1-lysine, respectively. Ly-
sine is not biologically available in these forms.
Acidic hydrolysis of such primary reaction
products yields lysine as well as the degradation
products furosine and pyridosine in a constant
ratio (cf. 4.2.4.4):

(1.147)

A nonreducing sugar (e. g. sucrose) can also
cause a loss of lysine when conditions for sugar
hydrolysis are favorable.
Losses of available lysine, cystine, serine, threo-
nine, arginine and some other amino acids occur
at higher pH values. Hydrolysates of alkali-
treated proteins often contain some unusual
compounds, such as ornithine,β-aminoalanine,
lysinoalanine, ornithinoalanine, lanthionine,
methyllanthionine andD-alloiso-leucine, as well
as otherD-amino acids.
The formation of these compounds is based on the
following reactions: 1,2-elimination in the case of
hydroxy amino acids and thio amino acids results
in 2-amino-acrylic acid (dehydroalanine) or 2-
aminocrotonic acid (dehydro-aminobutyric acid):

(1.148)

In the case of cystine, the eliminated thiolcysteine

can form a second dehydroalanine residue:

(1.149)

Alternatively, cleavage of the cystine disulfide

bond can occur by nucleophilic attack on sulfur,

yielding a dehydroalanine residue via thiol and

sulfinate intermediates:

(1.150)

(1.151)

(1.152)

Intra- and interchain cross-linking of proteins

can occur in dehydroalanine reactions involving

additions of amines and thiols. Ammonia may

also react via an addition reaction:

(1.153)

Acidic hydrolysis of such a cross-linked pro-

tein yields the unusual amino acids listed in