Contents Of Nucleotides In The Ingredients Of Foods

Biochemical Character of Muscles

The greater part of muscle fiber is made up of a set of specialized protein molecules that are in some way concerned with the process of muscular action. There is far less nucleic acid and very little ribonucleic acid in muscle tissue. The specialized molecules constitute about two-thirds of the protein, while the remaining one-third is concerned with the enzymatic use of the substances brought in by the blood capillaries that surround the fiber in the whole muscle.

In 1939 Engelhaldt and Ljubimowa discovered that myosin is an enzyme that catalyzes the conversion of ATP to the diphosphate ADP, with consequent release, or transfer, of configuration energy. More thorough studies by Banga and Szent-Gyorgyi (17) showed that two proteins were involved in myosin, namely, actin and myosin, which form a combination known as actomyosin, a combination capable of changing shape in a manner very reminiscent of the behavior of muscle itself. Additional studies have brought out that its molecular weight is 450,000, with an axial ratio of about 50 and a total length of 1600 A. That of actin is 60,000, with an axial ratio of 12 and a length of 290 A.

In the biochemistry of muscle, ATP, whose general structure is a triphosphate of adenine and ribose, is characterized by a phosphate grouping that, on hydrolysis, can yield a diphosphate and orthophosphoric acid, resulting in a more stable total configuration of the same atoms. This increase in stability means that energy is released when the process takes place.

Fish Muscle

In the muscle of fish, nucleotides such as adenosine ATP, ADP, AMP, and IMP have been detected. The changes in these nucleotides in muscle between the conditions of at rest and in fatigue are shown in Table 6, which summarizes the analysis of the codfish (18). In the condition of at rest, a large amount of ATP and quite a bit of adenosine and inosinic acid were found. When in fatigue, conversely, contents of inosinic acid increased. As already mentioned, the muscle released energy in the active state by the release of phosphate bonds from ATP, converting it into ADP, then AMP. This AMP is then changed to IMP by the aid of the enzyme deaminase. The procedure is as follows:

ATP ->• ADP + Pi by ATPase ADP ->■ AMP + Pi by myokinase AMP -» IMP + NH3 by deaminase

The autodigestion process occurs even after the death of animals; the change of ATP to AMP proceeds, and finally IMP is accumulated in the muscles. Therefore, meats in fatigue contain a high level of ATP in muscles so that, even after death, no conversion to IMP, yet occurred the taste of this supposed to be short of umami.

Fish meat after death contains mainly IMP, followed by inosine, which is decomposed to inosinic acid by the release of phosphoric acid; quite a bit of ADP and AMP remain. But Crustacea and Mollusca such as robster, crab, octopus, oysters, and cuttlefish kept in good condition for use as fresh meat have, on the contrary, a large amount of ADP or AMP and a trace of IMP because of a lack of the enzyme adenylic acid deaminase. Hence, there are two pathways of decomposition of adenylic acid in fish, shown as follows:

AMP deaminase IMP phosphatase ATP ADP ■ Adenylic acid ' Inosinic acid ■ Inosine Hypoxanthine + Ribose I AMP phosphatase t Adenosine deaminase Adenosine -

For Crustacea and Mollusca, adenylic acid is converted by AMP phosphatase in place of AMP daminase into adeno-

Table 6. Nucleotide Contents of Rested and Exhausted Codfish Muscle

Nucleotide

Content in muscle (wmoles/g)

Rested

Exhausted

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