92 Chapter 4
group of neutral cysteine proteinases that
upon activation, which involves cleavage
of the pro - domain and dimerization, cleave
proteins at specifi c aspartic acid residues
(Sentandreu et al. 2002 ; Fuentes - Prior and
Salvesen 2004 ; Herrera - Mendez et al. 2006 ).
To date there are 14 caspases that are divided
into three classes based on biological func-
tion: cytokine activators that function in
infl ammation, apoptosis initiator caspases,
and effector caspases (Fuentes - Prior and
Salvesen 2004 ; Herrera - Mendez et al. 2006 ).
During the apoptosis process, the initiator
caspases (caspases 8, 9, 10, and 12) activate
the downstream effector caspases (caspases
3, 6, and 7), which cleave specifi c target
proteins (Earnshaw et al. 1999 ). Since the
primary in vivo function of caspases is to
enzymatically degrade cellular structures
(Creagh and Martin 2001 ), in regards to meat
tenderization it has been postulated that
caspases would probably initially degrade
proteins involved in the spatial organization
of myofi brils and that further degradation
of cellular components would proceed with
the contribution of additional proteolytic
systems such as the calpains, cathepsins, and
proteasomes (Ouali et al. 2006 ). Similar
to calpains, caspases have been shown to
degrade a large number of muscle proteins
(Earnshaw et al. 1999 ; Nicholson 1999 ;
Fischer et al. 2003 ). In particular, caspase 3
has been shown to cleave myofi brillar pro-
teins in muscle during catabolic conditions
(Du et al. 2004 ).
Only a few studies, however, have inves-
tigated caspases in skeletal muscle in regards
to their potential contribution to postmortem
proteolysis and meat tenderization. Using
porcine trapezius , psoas , longissimus dorsi,
and semitendinosus muscle, it was demon-
strated that caspases and the caspase inhibitor
apoptosis repressor with caspase recruitment
domain (ARC) can be detected in different
muscle types at varying levels of expression
(Kemp et al. 2006a ). Incubation of recombi-
nant caspase 3 with porcine myofi brilsthat control postmortem proteolysis and
aging tenderization of meat.
Apoptosis Theory of Aging
Tenderization
Recent data has indirectly shown that the
process of apoptosis may play a role in post-
mortem proteolysis and meat tenderization
(Herrera - Mendez et al. 2006 ; Ouali et al.
2006 ). In living organisms, apoptosis, or pro-
grammed cell death, is a complex mechanism
by which cells can be eliminated without
damaging surrounding cells (Kerr et al. 1972 ;
Fidzianska et al. 1991 ). Apoptosis is initiated
and regulated by either the target cell or the
central nervous system, and is mediated by
the caspase enzyme system. As a result of the
slaughter process, the muscle tissue will be
deprived of oxygen and nutrients due to the
loss of the blood supply. The hypothesis is
that under these anoxic conditions, the muscle
cells will have no alternative but to initiate
apoptosis, which through the caspase system
would induce a series of biochemical and
structural changes important in the tenderiza-
tion process. Thus, the traditional model of
the conversion of muscle to meat would
include a phase corresponding to the initia-
tion of cell death in addition to the phases of
rigor mortis development and aging tender-
ization (Ouali et al. 2006 ). The apoptotic
process would then occur until muscle condi-
tions (pH, ionic strength, energy availability)
would be unfavorable for enzyme activity
(Ouali et al. 2006 ). Direct evidence to support
this emerging hypothesis is still lacking,
however.
Caspases
Apoptosis within the cell occurs through
the action of the caspase enzyme system.
Detailed information on the structure, activ-
ity, activation, and inhibition of caspases can
be found in the review of Fuentes - Prior and
Salvesen (2004). In brief, caspases are a