17 Seafood Enzymes 38920S Proteasome
The 20S proteasome enzyme is a 700 kDa multicat-
alytic proteinase with three major catalytic sites.
The term 20S refers to its sedimentation coefficient.
In the eukaryotic cells, the enzyme exists in the cy-
toplasm, either in a free state or associated with
large regulatory complexes. In vivo, it is involved in
nonlysosomal proteolysis and apoptosis. Research
strongly indicates that this proteasome is involved in
meat tenderization in cattle (Sentandreu et al. 2002).
Although proteasomes have been detected in the
muscle of such fish species as carp (Kinoshita et al.
1990a), white croaker (Busconi et al. 1992), and sal-
mon (Stoknes and Rustad 1995), their postmortem
activity in fish muscle has not been clarified.
Heat-Stable Alkaline Proteinases
Kinoshita et al. (1990b) reported the existence of
up to four distinct heat-stable alkaline proteinase
(HAP) in fish muscle: two sarcoplasmic proteinases
activated at 50 and 60°C, and two myofibril-associ-
ated proteinases activated at 50 and 60°C, respec-
tively. The distribution of the four proteinases was
found to be quite diverse among the 12 fish species
that were studied. The mechanisms activating these
proteinases in vivo and their precise physiological
functions are not clear.
The participation of one or the other protease in
the many different degradation scenarios that occur
has still been only partially elucidated. However,
various studies have found a close relationship be-
tween protein degradation in fish muscle and the ac-
tivity of specific proteases. A high level of activity
of cathepsin L has been found in chum salmon dur-
ing spawning, a period during which the fish exhibit
an extensive softening in texture (Yamashita and
Konagaya 1990). Similarly, Kubota et al. (2000)
found an increase in gelatinolytic activity in the
muscle of ayu during spawning, a period involving
a concurrent marked decrease in muscle firmness.
Also, in the muscle of hake, a considerably higher
level of proteolytic activity during the prespawning
period than in the postspawning period was found
(Perez-Borla et al. 2002).
The possible existence of a direct link between
protease activity and texture has been explored in
situ by perfusing protease inhibitors into fish muscle
and later measuring changes in texture during cold
storage. The activity of metalloproteinase and of
trypsin-like serine protease was found in this way to
play a role in the softening of flounder (Kubota et al.
2001) and of tilapia (Ishida et al. 2003).POSTMORTEM HYDROLYSIS OF
LIPIDS IN SEAFOOD DURING
FROZEN AND COLD STORAGEChanges in the lipid fraction of fish muscle during
storage can lead to changes in quality. Both the con-
tent and the composition of the lipids in fish muscle
can vary considerably between species and from one
time of year to another, and also differ greatly de-
pending upon whether white or red muscle fibers are
involved. As already mentioned, these two types of
muscle fibers are separated from each other, the
white fibers generally constituting most of the mus-
cle as a whole, although fish species vary consider-
ably in the amounts of dark meat, which has a high-
er myoglobin and lipid content. In species like tuna
and in small and fatty pelagic fish, the dark muscle
can constitute up to 48% of the muscle as a whole,
whereas in lean fish such as cod and flounder, the
dark muscle constitutes only a small percentage of
the muscle (Love 1970). Since triglycerides are de-
posited primarily in the dark muscle, providing fatty
acids as substrate to aerobic metabolism, whereas
the phospholipids represent most of the lipid frac-
tion of the white muscle, phospholipids constitute a
major part of the lipid fraction in lean fish (Lopez-
Amaya and Marangoni 2000b).
Not much research on lipid hydrolysis in fresh
fish during ice storage has been carried out, research
having concentrated more on changes in the lipid
fraction during frozen storage. This could be due to
freezing being the most common way of storing and
processing seafood and to lipid hydrolysis playing
no appreciable role in ice-stored fish before mi-
crobial spoilage becomes extreme. Knudsen (1989),
however, detected an increase in free fatty acids in
cod muscle during 11 days of ice storage, indicative
of the occurrence of lipolytic enzyme activity, an
increase that was most pronounced during days 5 to- Ohshima et al. (1984) reported a similar delay in
the increase in free fatty acids in cod muscle stored
in ice for 30 days. Both results are basically consis-
tent with the observation of Geromel and Mont-
gomery (1980) of no lysosomal lipase activity being
evident in trout muscle after seven days on ice. In