The primary taste component of chub mackerel is inosinic acid (IMP). Inosinic acid is generated by ATP degradation after fish death. However, it is degraded to non-taste components by IMP-degrading enzymes (IMPases) over time. Therefore, to maintain the taste of chub mackerel, IMPase activity should be inhibited. Chub mackerel is often processed using liquid seasoning including sugar. Honey is one of the sugar-rich products. As sugar has been reported to inhibit the activity of some types of enzymes, honey is also expected to inhibit IMPase activity. Therefore, in this study, we investigated the effects of honey, its main components (glucose and fructose), and its characteristic components (hydroxymethylfurfural (HMF) and melanoidin) on IMPase activity in chub mackerel. The results indicated that IMPase activity decreased with increasing honey concentration. Furthermore, a relatively high inhibitory effect of fructose was confirmed. Although HMF slightly inhibited IMPase activity, it did not decrease with increasing HMF concentration. In contrast, the IMPase activity decreased with increasing melanoidin concentration. These results indicate that honey has an inhibitory effect on IMPase activity in chub mackerel, and fructose and melanoidin are the main components inhibiting IMPase activity. Thus, honey could be an effective ingredient for processing chub mackerels.
Inosinic Acid, IMP Degrading Enzyme Activity, Fructose, Melanoidin, Maillard Reaction
Chub mackerel are small fish that migrate over a wide area and contribute to a good catch . Chub mackerel are consumed throughout the world because of their good taste. Similar to that, in other fish, the major taste component of chub mackerel is inosinic acid (IMP), which is generated by the degradation of ATP-related components as follows: ATP→ADP→AMP→IMP→HxR→Hx.
ATP is degraded to IMP relatively quickly, and then, IMP accumulates in the fish muscle . However, IMP is degraded to nontaste components inosine (HxR) and hypoxanthine (Hx) by IMPdegrading enzymes (IMPases) over time. Therefore, IMPase activity should be inhibited to maintain the taste of fish. IMPases consists some types of enzymes, and their composition and structure depend on fish species ; thus, IMPase in each fish species needs to be investigated. In addition, the conditions under which inhibit IMPase activity is inhibited in chub mackerel should also be investigated to preserve the taste.
Chub mackerel is often stored in liquid seasoning, brine, vinegar, miso, or sweet sake. Salt has been reported to inhibit IMPase activity in chub mackerel ; thus, processing methods using salt or liquid seasoning including salt can preserve the taste of chub mackerel. Sugar is often added to vinegar, miso, or sweet sake liquid seasoning. However, the effect of sugar on IMPase activity in chub mackerel has not been reported in detail. The main component of sugar is sucrose, which inhibits IMPase activity in the chicken grunt Parapristipoma trilineatum (Thunberg, 1793), Japanese amberjack Seriola quinqueradiata (Temminck & Schlegel, 1845), and Pacific cod Gadus macrocephalus (Tilesius, 1810) . Fructose has been reported to inhibit sucrase activity in rats ; thus, it is expected that these saccharides will also have an inhibitory effect on IMPase activity in chub mackerel. Honey contains glucose and fructose and is one of the sugar-rich products. Honey has been reported to inhibit acetylcholinesterase, which is associated with Alzheimer’s disease . It also inhibits the activity of angiotensin I-converting enzyme . Therefore, honey is expected to inhibit IMPase activity in chub mackerel. Hydroxymethylfurfural (HMF), which is generated by heating honey, has been reported to exert enhanced immunostimulatory effects . Melanoidin, the color component in honey, is reported to possess antioxidant activity . Furthermore, HMF inhibits an increase in the growth of ethanolgenerating yeast . Similarly, melanoidin inhibits carboxypeptidase , angiotensin I-converting enzyme, and lipase activities . Therefore, it is expected that these components could also inhibit IMPase in chub mackerel. To the best of our knowledge, there have been no studies on the inhibitory effect of honey on the activity of IMPase. Therefore, in this study, we aimed to examine the effect of honey and sugar on IMPase activity in chub mackerel. Furthermore, we investigated the effects of individual components of honey, HMF, and melanoidin on IMPase activity in chub mackerel.