2009_Book_FoodChemistry

12.3 Muscle Tissue: Composition and Function 581
A polypeptide chain with an aldehyde residue (I)
can interact with a lysine residue of the adjacent
chain to form an aldimine, which can be
further reduced to peptide-bound lysinonorleucine
(III):
(12.12)
If hydroxylysine is involved, an aldimine formed
initially can be converted to a more stable
β-aminoketone by Amadori rearrangement
(cf. 4.2.4.4.1):
(12.15)
Depending on the kind of condensation, in addition
to desmosine VI, designated as an A-type
condensation product, rings with other substitution
patterns are observed, i. e. B- and C-type condensation
products:
(12.13)
Likewise, aldehyde II can interact with one
lysine residue through the intermediary dehydromerodesmosine
(IV) to merodesmosine (V)
and, thus, provide cross-links between the three
adjacent polypeptide chains:
(12.16)
Pyridinolines have also been detected. They are
probably formed from β-aminoketones and the
ω-aldehyde of a hydroxylysine residue:
(12.14)
During the reaction of three aldehyde molecules
of type I with a lysine residue (actually a total of
four lysine side chains are involved), a pyridine
derivative is formed which, depending on the extent
of reduction, yields desmosine (VI), dihydro-
(VII) and tetrahydrodesmosine (VIII):
(12.17)
Studies of bovine muscle collagens have shown
that the pyridinoline content increases with increasing
age of the animal and, like the collagen
content, negatively correlates with the tenderness.
In intensively fattened cows, the pyridi-