Ikuo Kimura

Suppressive effect of ATP on autoxidation of tuna oxymyoglobin to metmyoglobin

The discoloration of tuna meat proceeds during frozen storage at around −20xa0°C. On the other hand, the discoloration of highly fresh tuna meat could effectively be suppressed even if stored at −20xa0°C. However, the suppressive mechanism of the discoloration is not well understood. Here the effects of ATP on the autoxidation rate and molecular structure of tuna myoglobin are reported. The autoxidation rate of southern bluefin tuna Thunnus maccoyii myoglobin at 25xa0°C was suppressed in the presence of ATP especially in acidic pH range. Mixing ATP with myoglobin induced a spectral perturbation in the soret region of myoglobin. This spectral perturbation was observed as a function of the ATP concentration. Quenching of myoglobin fluorescence was also caused by ATP, saturating at around 0.5xa0mM ATP. According to dynamic light-scattering measurements, the molecular weights of tuna Mb changed from 15.5 to 11.3xa0kDa with ATP and zeta-potential measurements gave also a negative surface charge without ATP and a positive one with ATP, respectively. The above results indicate that ATP-induces changes in the conformational structure of myoglobin. The effects of ATP on myoglobin could thus provide a possible mechanism to regulate the autoxidation of myoglobin.

Suppressive effect of ATP on the denaturation of sarcoplasmic reticulum (Ca2+)-ATPase from southern bluefin tuna Thunnus maccoyii and its biochemical properties

Sarcoplasmic reticulum (SR) was prepared from ordinary white muscle of southern bluefin tuna. The chymotryptic digestibility, pH and temperature dependent Ca2+-ATPase activity of SR, as well as the thermal and freezing stability with different additives including ATP were examined. A major component of SR Ca2+-ATPase, which was about 100xa0kDa by SDS-PAGE analysis, was digested to 60xa0kDa and 38xa0kDa fragments by α-chymotrypsin. The SR Ca2+-ATPase activity was 0.3xa0μmol Pi/min/mg at 25xa0°C. There was no break point in the Arrhenius plot of temperature-dependent Ca2+-ATPase activity. The activation energy of Ca2+-ATPase was 35.44xa0kJ/mol. The pH dependence of SR Ca2+-ATPase activity of southern bluefin tuna showed a clear peak with a maximum activity at pH 6.5, and SR Ca2+-ATPase was half inactivated when heated at 45xa0°C for 4xa0min. SR Ca2+-ATPase was almost inactivated by pH 5.5 acid treatment, for 2xa0h at 0xa0°C. Denaturation by acid treatment was strongly suppressed by the addition of 2xa0mM ATP. ATP also showed a strong suppressive effect against denaturation of SR Ca2+-ATPase by freezing and heating. These results indicate that ATP reduces denaturation of SR Ca2+-ATPase and that enhancement of ATP levels during freezing has the potential to improve meat quality.