14 Biochemistry of Processing Meat and Poultry 317The stability of calpain I in postmortem muscle is
very poor because it is readily autolyzed, especially
at high temperatures, in the presence of the released
Ca^2 (Koohmaraie 1994). Calpain II appears as more
stable, just 2–3 weeks before losing its activity
(Koohmaraie et al. 1987). In view of this rather poor
stability, the importance of calpains should be re-
stricted to short-term processes. A minor contribu-
tion, just at the beginning, has been observed in long
processes such as dry curing of hams (Rosell and
Toldrá 1996) or in fermented meats where the acid
pH values makes calpain activity rather unlikely
(Toldrá et al. 2001).
Calpastatin is a polypeptide (between 50 and 172
kDa) acting as an endogenous reversible and com-
petitive inhibitor of calpain in the living muscle. In
postmortem muscle, calpastatin regulates the activ-
ity of calpains, through a calcium dependent interac-
tion, although only for a few days, because it is de-
stroyed by autolysis (Koohmaraie et al. 1987). The
levels of calpastatin vary with animal species, and
pork muscle has the lowest level (Valin and Ouali
1992).
Lysosomal Proteinases: Cathepsins
There are several acid proteinases in the lysosomes
that degrade proteins in a nonselective way. The
most important are cathepsins B, H, and L, which
are cysteine proteinases, and cathepsin D, which is
an aspartate proteinase. The optimal pH for activity
is slightly acid (pH around 6.0) for cathepsins B and
L, acid (pH around 4.5) for cathepsin D, and neutral
(pH 6.8) for cathepsin H (Toldrá et al. 1992). Cath-
epsins require a reducing environment such as that
found in postmortem muscle to express their opti-
mal activity (Etherington 1987). All of them are of
small size, within the range 20–40 kDa, and are thus
able to penetrate into the myofibrillar structure.
Cathepsins have shown a good ability to degrade
different myofibrillar proteins. Cathepsins D and L
are very active against myosin heavy chain, titin, M
and C proteins, tropomyosin, and troponins T and I
(Matsakura et al. 1981, Zeece and Katoh 1989).
Cathepsin L is extremely active in degrading both
titin and nebulin. Cathepsin B is able to degrade
myosin heavy chain and actin (Schwartz and Bird
1977). Cathepsin H exhibits both endo- and amino-
peptidase activity, and this is the reason for its clas-
sification as an aminoendopeptidase (Okitani et al.
1981). In the muscle, there are endogenous inhib-
itors against cysteine peptidases. These inhibitory
compounds, known as cystatins, are able to inhibit
cathepsins B, H, and L. Cystatin C and chicken cys-
tatin are the most well known cystatins.Proteasome ComplexThe proteasome is a multicatalytic complex with
different functions in living muscle, even though its
role in postmortem muscle is still not well under-
stood. The 20S proteasome has a large molecular
mass, 700 kDa, and a cylinder-shaped structure with
several subunits. Its activity is optimal at pH above
7.0, but it rapidly decreases when pH decreases,
especially below 5.5. It exhibits three major activi-
ties: chymotrypsin-like activity, trypsin-like activity,
and peptidyl-glutamyl hydrolyzing activity (Coux et
al. 1996). This multiple activity behavior is the rea-
son why there was originally some confusion among
laboratories over its identification. The 20S protea-
some concentration is higher in oxidative muscles
than in glycolytic ones (Dahlman et al. 2001). This
enzyme has shown degradation of some myofibril-
lar proteins such as troponin C and myosin light
chain and could be involved in postmortem changes
in slow twitch oxidative muscles or in high pH meat,
where an enlargement of the Z-line with more
or less density loss is observed (Sentandreu et al.
2003).Exoproteases: PeptidasesThere are several peptidases in the muscle with the
ability to release small peptides of importance for
taste. Tripeptidylpeptidases (TPPs) are enzymes cap-
able of hydrolyzing different tripeptides from the
amino termini of peptides, while Dipeptidylpeptid-
ases (DPPs) are able to hydrolyze different dipeptide
sequences. There are two TPPs and four DPPs, and
their molecular masses are relatively high, between
100 and 200 kDa, or even as high as 1000 kDa for
TPP II, and have different substrate specificities.
TPP I is located in the lysosomes, has an optimal
acid pH (4.0), and is able to hydrolyze tripeptides
Gly-Pro-X, where X is an amino acid, preferentially
of hydrophobic nature. TPP II has optimal neutral
pH (6.5–7.5) and wide substrate specificity, except
when Pro is present on one of both sides of the
hydrolyzed bond. DPPs I and II are located in the