魚類筋肉タンパク質の温度およびpH依存性に見られる特性に関する研究

1. Characteristics of Temperature Dependence of Fish Muscle Protein Since fish adapts to various environmental water temperatures ranging from -2 to 40°C, it is very interesting to clarify the relationship between the properties of these muscle proteins and the temperature. Therefore, using the myofibrils which are the model system of the muscle, the temperature characteristics and the stability against heat of its biochemical function were compared between the cold, temperate and tropical fishes. 1. Mg-ATPase (I = 0.12, pH 7.0) activity of fish myofibrils was activated when Ca concentration was high (0.25 mM), but was inhibited when it was low (<10 nM). The Ca-sensitivity (%) calculated from these two ATPase activities showed a sufficiently high value of 90 to 99% near the inhabitation temperature range of the fish. 2. As the ATPase measurement temperature was further raised, the Ca-sensitivity decreased irreversibly: at >25°C in cold-water fish, at >35°C in temperate fish, and at >40°C in tropical fish. The stability of myofibrils to heat is different in these fish species. 3. For 40 fish species, the thermal stability of myofibrils was compared kinetically using first order rate constants (KD) in inactivation of the Ca-ATPase activity. The value of activation free energy in denaturation, ⊿ G≠(kcal/mol), was found to be closely related to the favorable environmental water temperature, t (°C) of the fish; it was expressed by the equation of ΔG≠ = 22.45 + 0.1744 t (r=0.950). 2. Characteristics of PH Dependence of Fish Muscle Protein In fish meat after death, pH decrease due to accumulation of lactic acid occurs. This tendency is remarkable in red-fleshed fish and it may be a cause of quality deterioration of fish meat after catching. In Part 2, the influence of acidic pH ( 5.5- 6.5 ) on the stability of fish myofibrils was studied at various temperatures. Furthermore, the mechanism of acid denaturation was also investigated. 1. As the pH decreased from 7.0 to 5.5 at constant temperature, inactivation rate (KD) for fish myofibrillar Ca-ATPase increased markedly. 2. KD of myofibrils of sardine and mackerel was measured at pH5.8 in the range of 3-40°C. The Arrhenius Plot (plot of the log KD against 1/T °K) showed a transition break at around 20°C: the activation enthalpy(Δ H≠ ) was 13-26 kcal/mol in the low temperature region and 50-60 kcal/mol in the high temperature region. The former value is about 1/2 to 1/4 of the latter value. Preventing myofibril denaturation, it may be effective for the method of cooling in the high temperature range and the method to neutralize the pH in the low temperature range. 3. The mechanism of acid denaturation of rabbit reconstituted actomyosin was investigated at pH 5.5(I=0.5) in both at 35°C and at 3°C. As a result, it was suggested that denaturation of actomyosin is accompanied mainly by aggregation reaction of the molecule in the high temperature region, while release of myosin by denaturation of actin and structural change of actomyosin mainly occurs in the low temperature range. 3. Effect of PH and Temperature on Gel forming Properties of Fish Muscle Protein Fish meat changes to the elastic gel-like food through washing with water, grinding with salt, shaping, pre-heating and heating. Therefore, its gelforming ability may be greatly influenced by the characteristics of fish myofibrillar protein. In part3, based on the study of denaturation for fish myofibrils in Part1 and Part2, the characteristics of gel formation were also investigated using minced fish flesh or “surimi”. 1. The gel forming ability of “surimi” with or without salt was quickly lost as the storage temperature was higher. First order rate constants (KJ) in the decrease of gel forming ability were considerably different among fishes: Alaska Pollack (cold-water fish) > White croaker (temperate fish) > Tilapia (tropical fish). This trend well corresponded to that of myofibrillar thermal stability among fish species in Part1. 2. “Surimi” of each fish species was grinded with salt and stored at 10-35°C, and changes in gel strength were recorded. Comparing the rate of gelation among fish species, it was faster in the order of Alaska Pollack (cold-water fish) > White croaker (temperate fish) > Tilapia (tropical fish). This result similary reflected the thermal stability between fish species. 3. The influence of weakly acid pH on the gel forming ability of “surimi” was investigated. The gel forming ability was largely lost as the time of acid treatment was longer and the pH was lower. To summarize these results, fish myofibrillar protein retains thermal stability closely related to the fish's habitat temperature, so when storing and processing cold-water and tropical fish muscles, suggesting that the processing conditions differ for each fish species. Furthermore, in the effective utilization of migratory red-meat fish, in addition to thermal denaturation factors, PH reduction of muscle after death is also a degeneration factor, so there is a need to neutralize the pH of the meat as soon as possible after catching. This study showed that properly controlling the mode and speed of denaturation is necessary for highly using muscles of various fish as food by grasping the temperature and pH characteristics of muscle proteins of each fish.