2009_Book_FoodChemistry

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. Lysine
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):
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)
(1.147)
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, threonine,
arginine and some other amino acids occur
at higher pH values. Hydrolysates of alkalitreated
proteins often contain some unusual
compounds, such as ornithine, β-aminoalanine,
lysinoalanine, ornithinoalanine, lanthionine,
methyllanthionine and D-alloiso-leucine, as well
as other D-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):
(1.151)
(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)
(1.148)
Acidic hydrolysis of such a cross-linked protein
yields the unusual amino acids listed in