17 Seafood Enzymes 381waters. Several lines of evidence suggest that the
decrease in metabolic rate with increasing depth
is largely due to the food consumption levels being
lower and to a reduced need for locomotor capabili-
ty in the darkness (Childress 1995, Gibbs 1997).
The latter makes intuitive sense if one considers the
often far from streamlined, and sometimes bizarre,
anatomy of deep-sea fish species.
OSMOTICADAPTATION OFENZYMES
Marine teleosts (bonefish) are hypoosmotic, having
intracellular solute concentrations that are similar to
those of terrestrial vertebrates. By contrast, elasmo-
branches (sharks and rays) are isoosmotic, employ-
ing the organic compounds urea and TMAO as osmo-
lytes. Although the concentration of osmolytes can
be high, the stability and functionality of enzymes
remain mostly unaffected due to the compatability
and balanced counteracting effects of the osmolytes
(Yancey et al. 1982).
ENZYMATIC REACTIONS IN THE
ENERGY METABOLISM OF
SEAFOOD
Most key metabolic pathways such as the glycolysis
are found in all animal species, and the majority of
animal enzymes are thus homologous. Nevertheless,
the energy metabolism of seafood species shows a
number of differences from that of terrestrial ani-
mals, and these differences contribute to the charac-
teristics of seafood products.
The red and white fish muscle cells are anatomi-
cally separated into minor portions of red muscle
and major portions of white muscle tissues (see Fig.
17.2 in the section about postmortem proteolysis in
fresh fish). The energy metabolism and the histology
of the two muscle tissues differ, the anaerobic white
muscle having only few mitochondria and blood
vessels. When blood circulation stops after death,
the primary effect on the white muscle metabolism
is therefore not the seizure of oxygen supply but the
accumulation of waste products that are no longer
removed.
Adenosine-5’-triphosphate (ATP) is the predomi-
nant nucleotide in the muscle cells of rested fish and
represents a readily available source of metabolic
energy. It regulates numerous biochemical processes
and continues to do so during postmortem process-
es. Both the formation and degradation of ATP con-
tinue postmortem, the net ATP level being the result
of the difference between the rates of the two
processes. In the muscle of rested fish, the ATP con-
tent averages 7–10 μmol/g tissue (Cappeln et al.
1999, Gill 2000). This concentration is low in com-
parison with the turnover of ATP, and the ATP pool
would thus be quickly depleted if not continously
replenished.POSTMORTEMGENERATION OFAT PIn anaerobic tissues such as postmortem muscle
tissue, ATP can be generated from glycogen by gly-
colysis. Smaller amounts of ATP may also be gener-
ated by other substances, including creatine or argi-
nine phosphate, glucose, and adenosine diphosphate
(ADP).
Creatine phosphate provides an energy store that
can be converted into ATP by a reversible transfer of
a high-energy phosphoryl group to ADP catalyzed
by sarcoplasmic creatine kinase (EC 2.7.3.2). The
creatine phosphate in fish is usually depleted within
hours after death (Chiba et al. 1991). In many inver-
tebrates, including shellfish, creatine phosphate is
often substituted by arginine phosphate, and ATP is
thus generated by arginine kinase (EC 2.7.3.3) (Ell-
ington 2001, Takeuchi 2004).
Glucose can enter glycolysis after being phospho-
rylated by hexokinase (EC 2.7.1.1). Although glu-
cose represents an important energy resource in liv-
ing animals, it is in scarce supply in postmortem
muscle tissue due to the lack of blood circulation,
and it contributes only little to the postmortem for-
mation of ATP.
ATP is also formed by the enzyme adenylate
kinase (EC 2.7.4.3), which catalyzes the transfer
of a phosphoryl group between two molecules of
ADP, producing ATP and adenosine monophosphate
(AMP). As AMP is formed, adenylate kinase parti-
cipates also in the degradation of adenine nucleo-
tides, as will be described later.
The glycogen deposits in fish muscle are generally
smaller than the glycogen stores in the muscle of rest-
ed mammals. Nevertheless, muscle glycogen is nor-
mally responsible for most of the postmortem ATP
formation in fish. The degradation of glycogen and
the glycolytic formation of ATP and lactate proceed
by pathways that are similar to those in mammals.
Glycogen is degraded by glycogen phosphorylase