414 Part III: Muscle Foods
although further studies are needed to verify that
statement.
Potential for Further Antemortem Protein
Degradation Studies
We are not aware of any proteomic studies, other
than that discussed above, into the link between an-
temortem protein metabolism and postmortem qual-
ity in fish and seafood. However, given the substan-
tial importance of protein degradation to the quality
and processability of fish and seafood, it may be
worthwhile to consider the potential for application
of proteomics within this field of study. In addition
to having a hand in controlling autolysis determi-
nants, protein turnover is a major regulatory engine
of cellular structure, function, and biochemistry.
Cellular protein turnover involves at least two major
systems: the lysosomal system and the ubiquitin-
proteasome system (Hershko and Ciechanover 1986,
Mortimore et al. 1989). The 20S proteasome has
been found to have a role in regulating the efficiency
with which rainbow trout deposit protein (Dobly et
al. 2004). It seems likely that the manner in which
protein deposition is regulated, particularly in mus-
cle tissue, has profound implications for the quality
and processability of the fish flesh.
Protein turnover systems, such as the ubiquitin-
proteasome or the lysosome systems, are suitable for
rigorous investigation using proteomic methods. For
example, lysosomes can be isolated and the lyso-
some subproteome queried to answer the question of
whether and to what extent lysosome composition
varies among fish expected to yield flesh of different
quality characteristics. Proteomic analysis on lyso-
somes has been successfully performed in mam-
malian (human) systems (Journet et al. 2000, Journet
et al. 2002).
An exploitable property of proteasome-mediated
protein degradation is the phenomenon of polyubiq-
uitination, whereby proteins are targeted for destruc-
tion by the proteasome by covalent binding to multi-
ple copies of ubiquitin (Ciechanover 1994, Hershko
and Ciechanover 1986). By targeting these ubiquitin-
labeled proteins, it is possible to observe the ubiquitin-
proteasome “degradome,” that is, to observe which
proteins are being degraded by the proteasome at a
given time or under given conditions. Peng et al.
have developed methods to study the ubiquitin-pro-
teasome degradome in the yeast Saccharomyces
cerevisiaeusing multidimensional LC-MS/MS (Peng
et al. 2003).
Some proteolysis systems, such as that of the
matrix metalloproteases, may be less directly amen-
able to proteomic study. Activity of matrix metallo-
proteases is regulated via a complex network of spe-
cific proteases (Brown et al. 1993, Okumura et al.
1997, Wang and Lakatta 2002). Monitoring of the
expression levels of these regulatory enzymes and
how they vary with environmental or dietary vari-
ables may be more conveniently carried out using
transcriptomic methods.Can Antemortem Proteomics Shed Light on
Gaping Tendency?A well-known quality issue when farmed fish are
compared with wild catch is that of gaping, a phe-
nomenon caused by cleavage by matrix proteases of
myocommatal collagen cross-links, which results in
weakening and rupturing of connective tissue (Bør-
resen 1992, Foegeding et al. 1996). Gaping can be a
serious quality issue in the fish processing industry
as, apart from the obvious visual defect, it causes
difficulties in mechanical skinning and slicing (Love
1992) of the fish. Weakening of collagen, and hence,
gaping, is facilitated by low pH. Well-fed fish, such
as those reared in aquaculture, tend to yield flesh of
comparatively low pH, which thus tends to gape
(Einen et al. 1999, Foegeding et al. 1996). Gaping is
therefore a cause for concern with aquaculture-
reared fish, particularly of species with high natural
gaping tendency, such as the Atlantic cod. Gaping
tendency varies considerably among wild fish
caught in different areas (Love et al. 1974), and thus,
it is conceivable that gaping tendency can be con-
trolled with dietary or other environmental manip-
ulations. Proteomics and transcriptomics, with their
capacity to monitor multiple biochemical processes
simultaneously, are methodologies eminently suit-
able to finding biochemical or metabolic markers
that can be used for predicting features such as gap-
ing tendendency of different stocks reared under dif-
ferent dietary or environmental conditions.SPECIES AUTHENTICATION
Food authentication is an area of increasing impor-
tance, both economically and from a public health
standpoint. Taking into account the large differences