BLBS102-c13 BLBS102-Simpson March 21, 2012 13:15 Trim: 276mm X 219mm Printer Name: Yet to Come
13 Seafood Enzymes 249Adenosine-5′-triphosphate (ATP) is the predominant nu-
cleotide in the muscle cells of rested fish and represents a readily
available source of metabolic energy. It regulates numerous bio-
chemical processes and continues to do so during postmortem
processes. Both the formation and degradation of ATP continue
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 content averages 7–10μmol/g tissue (Cappeln
et al. 1999, Gill 2000). This concentration is low in compari-
son with the turnover of ATP, and the ATP pool would thus be
quickly depleted if not continuously replenished.Postmortem Generation of ATPIn anaerobic tissues such as postmortem muscle tissue, ATP can
be generated from glycogen by glycolysis. Smaller amounts of
ATP may also be generated by other substances, including crea-
tine or arginine phosphate, glucose, and adenosine diphosphate
(ADP).
Creatine phosphate provides an energy store that can be con-
verted into ATP by a reversible transfer of a high-energy phos-
phoryl group to ADP catalyzed by sarcoplasmic creatine kinase
Enzyme Commision number (EC 2.7.3.2). The creatine phos-
phate in fish is usually depleted within hours after death (Chiba
et al. 1991). In many invertebrates, including shellfish, creatine
phosphate is often substituted by arginine phosphate, and ATP is
thus generated by arginine kinase (EC 2.7.3.3) (Ellington 2001,
Takeuchi 2004).
Glucose can enter glycolysis after being phosphorylated by
hexokinase (EC 2.7.1.1). Although glucose represents an impor-
tant energy resource in living 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 formation 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 nucleotides, as
will be described later.
The glycogen deposits in fish muscle are generally smaller
than the glycogen stores in the muscle of rested mammals. Nev-
ertheless, muscle glycogen is normally 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 (EC 2.4.1.1),
producing glucose-1-phosphate. The highly regulated glycogen
phosphorylase is inhibited by glucose-1-phosphate and glu-
cose and is activated by AMP. Glucose-1-phosphate is subse-
quently transformed into glucose-6-phosphate by phosphoglu-
comutase (EC 5.4.2.2). Glucose-6-phosphate is further degraded
by the glycolysis, leading to the formation of ATP, pyruvate, and
Nicotineamide adenine dinucleotide (NADH). NADH originates
from the reduction of NAD+during glycolysis. In the absence
of oxygen, NAD+is regenerated by LDH (EC 1.1.1.27), as
pyruvate is reduced to lactate. The formation of lactate forms
a metabolic blind end and causes pH to fall, the final pH levelbeing reached when the formation of lactate has stopped. The
postmortem concentration of lactate reflects, by and large, the to-
tal degradation of ATP. Since in living fish, however, removal of
lactate from the large white muscle often proceeds only slowly,
most lactate formed just before death may remain in the muscle
postmortem. Thus, the final pH level in the fish is affected only a
little by the stress to which the fish was exposed and the struggle
it went through during capture (Foegeding et al. 1996).
The accumulation of lactate in fish is limited by the amount
of glycogen in the muscle. Since glycogen is present in fish
muscle at only low concentrations, the final pH level of the fish
meat is high in comparison to that of other types of meat, typ-
ically 6.5–6.7. In species that have more muscular glycogen,
this substance rarely limits the formation of lactate, which may
continue until the catalysis stops for other reasons, at a pH of
approximately 5.5–5.6 in beef (Hultin 1984). The final pH de-
pends not only on the lactate concentration, but also on the pH
buffer capacity of the tissue. In the neutral and slightly acidic
pH range, the buffer capacity of the muscle tissue in most an-
imals, including fish, originates primarily from histidyl amino
acid residues and phosphate groups (Somero 1981, Okuma and
Abe 1992). Some fish species, in order to decrease muscular
acidosis, have evolved particularly high pH buffer capacities.
For example, pelagic fish belonging to the tuna family (genus
Thunnus) possess a very high glycolytic capacity (Storey 1992,
and references therein) and also accumulate high concentrations
of histidine and anserine (N-(β-alanyl)-N-methylhistidine) (van
Waarde 1988, Dickson 1995). Since tuna and a few other fish
species can effectively retain metabolic heat, their core temper-
ature at the time of capture may significantly exceed the water
temperature. The high muscle temperature of such species ex-
pectedly accelerates autolytic processes, including proteolysis,
and may, in combination with the lowering of pH, promote pro-
tein denaturation (Haard 2002).
Some species of carp, including goldfish, are able to survive
for several months without oxygen. The metabolic adaptation
this requires includes substituting ethanol for lactate as the ma-
jor end product of glycolysis. Ethanol has the advantage over
lactate that it can be easily excreted by the gills and does not lead
to acidosis (Shoubridge and Hochachka 1980). Under anoxic
conditions, NAD+is thus regenerated by muscular alcohol de-
hydrogenase (EC 1.1.1.1).Postmortem Degradation of NucleotidesATP can be metabolized by various ATPases. In muscle tissues,
most of the ATPase activity is represented by Ca^2 +-dependent
myosin ATPase (EC 3.6.1.32) that is directly involved in contrac-
tion of the myofibrils. Thus, release of Ca^2 +from its containment
in the sarcoplasmic reticulum dramatically increases the rate of
ATP breakdown.
The glycogen consumption during and after slaughter is
highly dependent on the fishing technique and the slaughter-
ing process employed. In relaxed muscle cells, the sarcoplasmic
Ca^2 +levels are low, but upon activation by the neuromuscu-
lar junctions, Ca^2 +is released from the sarcoplasmic reticulum,
activating myosin ATPase. Since the muscular work performed