Food Chemistry

12.3 Muscle Tissue: Composition and Function 581

A polypeptide chain with an aldehyde residue (I)
can interact with a lysine residue of the adja-
cent chain to form an aldimine, which can be
further reduced to peptide-bound lysinonor-
leucine (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):

(12.13)

Likewise, aldehyde II can interact with one
lysine residue through the intermediary dehy-
dromerodesmosine (IV) to merodesmosine (V)
and, thus, provide cross-links between the three
adjacent polypeptide chains:

(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 ex-
tent of reduction, yields desmosine (VI), dihydro-
(VII) and tetrahydrodesmosine (VIII):

(12.15)

Depending on the kind of condensation, in add-

ition to desmosine VI, designated as an A-type

condensation product, rings with other substitu-

tion patterns are observed, i. e. B- and C-type con-

densation products:

(12.16)

Pyridinolines have also been detected. They are

probably formed fromβ-aminoketones and the

ω-aldehyde of a hydroxylysine residue:

(12.17)

Studies of bovine muscle collagens have shown

that the pyridinoline content increases with in-

creasing age of the animal and, like the colla-

gen content, negatively correlates with the ten-

derness. In intensively fattened cows, the pyridi-