Food Biochemistry and Food Processing

17 Seafood Enzymes 383

of ADP is favored by the removal of AMP by AMP
deaminase (EC 3.5.4.6), producing inosine mono-
phosphate (IMP) and ammonia. AMP deaminase is a
key regulator of the intracellular adenine nucleotide
pool and as such is a highly regulated enzyme inhib-
ited by inorganic phosphate, IMP and NH 3 and acti-
vated by ATP.
It is widely accepted that IMP contributes to the
desirable taste of fresh seafood (Fluke 1994, Gill
2000, Haard 2002). IMP is present in small amounts
in freshly caught relaxed fish but builds up during
the depletion of ATP (Berg et al. 1997). IMP can be
hydrolyzed to form inosine, yet the rate of hydroly-
sis in fish is generally low, following zero-order kin-
etics, indicating that the IMP-degrading enzymes
are fully substrate saturated (Tomioka et al. 1987,
Gill 2000, Itoh and Kimura 2002). However, a large
species variation in the degradation of IMP has been
shown, and IMP may thus persist during storage for
several days or even weeks (Dingle and Hines 1971,
Gill 2000, Haard 2002). The formation of inosine is
often rate limiting for the overall breakdown of

nucleotides and is considered to constitute the last

purely autolytic step in the nucleotide catabolism of

chilled fish. The formation of inosine usually indi-

cates a decline in the prime quality of seafood

(Surette et al. 1988, Gill 2000, Haard 2002). IMP can

be hydrolyzed by various enzymes, 5’-nucleotidase

(EC.3.1.3.5) being regarded as the most important

of these in the case of chilled fish (Gill 2000, Haard

2002). Several forms of 5’-nucleotidase exist, of

which a soluble sarcoplasmic form has been docu-

mented in fish (Itoh and Kimura 2002).

The degradation of inosine continues via hypox-

anthine and its oxidized products xanthine and uric

acid. These reactions, all of them related to spoilage,

are catalyzed by both endogenous and microbial

enzymes, the activity of the latter depending upon

the spoilage flora that are present and thus differing

much between species and products. Inosine can

be degraded by either nucleoside phosphorylase

(EC 2.4.2.1) or inosine nucleosidase (EC 3.2.2.2),

leading to the formation of different by-products

(see Fig. 17.1). The final two-step oxidation of

Figure 17.1.The successive reactions of the main pathway for nucleotide degradation in fish. Substances listed
below the dotted line are generally considered to be indicators of spoilage. Piis inorganic phosphate.