Lysine (Lys) and hydroxylysine (Hyl) are present in the helical
and in the non-helical region of the collagen molecule. Both amino
acids form intermolecular non-hydrolysable bonds in the collagen,
giving rise to cross-links (Asghar & Henrickson, 1982; Li et al.,
2005). The presence of Hyl was significantly greater in the FP than
in the WP collagen (Table 2), indicating that the FP collagen was
more cross-linked than the WP collagen. However, the content of
Lys residues was significantly greater in the WP than in FP. Lys is
involved in the formation of non-reducible groups called hydroxylysyl
pyridinoline (HP) and lysyl pyridinoline (LP) that can connect
three collagen molecules (Li et al., 2005), thus inducing collagen
inter-molecular cross-linking and enhanced connective tissue
strength. The greater firmness of the WP group might, therefore,
be associated with the greater connective tissue content concomitant
with the greater Lys content compared with the FP group.