14 Chapter 1
myofi brils (Huff - Lonergan et al. 1996a ;
Huff - Lonergan and Lonergan, 1999 ; Carlin
et al. 2006 ). Thus, the proteolytic enzyme
μ - calpain may be, at least in part, responsible
for desmin degradation under normal post-
mortem aging conditions. Whether or not this
degradation is truly directly linked to tender-
ization or is simply an indicator of overall
postmortem proteolysis remains to be
determined.Filamin
Filamin is a large ( M r = 245,000 in skeletal
and cardiac muscle) actin - binding protein
that exists in numerous cell types (Loo et al.
1998 ; Thompson et al. 2000 ; van der Flier et
al. 2002 ). There are several different iso-
forms of fi lamin (Hock et al. 1990 ). The
amount of fi lamin in skeletal and cardiac
muscle is very low (approximately ≤ 0.1% of
the total muscle protein). In skeletal and
cardiac muscle, fi lamin is localized at the
periphery of the myofi brillar Z - disk, and it
may be associated with intermediate fi la-
ments in these regions (Loo et al. 1998 ;
Thompson et al. 2000 ; van der Flier et al.
2002 ). Thus, postmortem degradation of
fi lamin conceivably could disrupt key link-
ages that serve to help hold myofi brils in
lateral register. Degradation of fi lamin may
also alter linkages connecting the peripheral
layer of myofi brils in muscle cells to the sar-
colemma by weakening interactions between
peripheral myofi brillar Z - disks and the sarco-
lemma via intermediate fi lament associations
or costameres (Robson et al. 1995 ). A study
using myofi brils from beef showed that some
fi lamin was degraded to form an approxi-
mately 240 - kDa degradation product that
migrated as a doublet in both myofi brils from
naturally aged muscle and in μ - calpain -
digested myofi brils (Huff - Lonergan et al.
1996a ). This same doublet formation (com-
posed of intact and degraded fi lamin) has
been seen in cultured embryonic skeletal
muscle cells and was attributed to calpainof polypeptides in the 30 - kDa region seem to
be a valuable indicator of beef tenderness
(Olson et al. 1977 ; Olson and Parrish, 1977 ;
Koohmaraie et al. 1984a, b ; Koohmaraie
1992 ; Huff - Lonergan et al. 1995 ; Huff -
Lonergan et al. 1996a ; Huff - Lonergan and
Lonergan 1999 ).
Desmin
It has been suggested that desmin, an inter-
mediate fi lament protein (O ’ Shea et al. 1979 ;
Robson 1989 ) localized at the periphery of
the myofi brillar Z - disk in skeletal muscle
(Richardson et al. 1981 ), plays a role in the
development of tenderness (Taylor et al.
1995 ; Huff - Lonergan et al. 1996a ; Boehm et
al. 1998 ; Melody et al. 2004 ). The desmin
intermediate fi laments surround the Z - lines
of myofi brils. They connect adjacent myofi -
brils at the level of their Z - lines, and the
myofi brils to other cellular structures, includ-
ing the sarcolemma (Robson, 1989 ; Robson
et al. 1995 ). Desmin may be important in
maintaining the structural integrity of muscle
cells (Robson et al. 1981, 1991 ). It is possible
that degradation of structural elements that
connect the major components (i.e., the myo-
fi brils) of a muscle cell together, as well as
the peripheral layer of myofi brils to the cell
membrane, could affect the development of
tenderness. Desmin is degraded during post-
mortem storage (Hwan and Bandman 1989 ;
Huff - Lonergan et al. 1996a ; Huff - Lonergan
and Lonergan, 1999 ; Melody et al. 2004 ;
Rowe et al. 2004b ; Zhang et al. 2006 ).
Furthermore, it has been documented that
desmin is degraded more rapidly in myofi -
brils from samples with low shear force
and higher water - holding capacity (Huff -
Lonergan et al. 1996a ; Huff - Lonergan and
Lonergan, 1999 ; Melody et al. 2004 ; Rowe
et al. 2004b ; Zhang et al. 2006 ). A major
degradation product that is often seen in beef
is a polypeptide of approximately 38 kDa.
This degradation product also has been
shown to be present in μ - calpain - digested