SUMMARY: Bluefin tuna (Thunnus thynnus) meat, wrapped with or without polyethylene film, was stored at 4°C and relative humidity of 100%. Decrease in Hunter’s ‘a’ and increase in ‘L’ were observed in both samples during 120 h storage. Metmyoglobin (metMb) reductase activity of both samples decreased, while the metMb increased with the duration of storage. The nicotinamide adenine dinucleotide (NADH) of tuna meat increased during the first 56–80 h of storage, but decreased again during the further storage. Although the nicotinamide adenine dinucleotide phosphate (NADPH) of both wrapped and unwrapped samples decreased with the duration of storage, the total amount of NADH and NADPH seemed to remain at a constant level during the first 80 h storage and then significantly decreased during further storage. The metMb reductase is an important factor affecting the accumulation of metMb of refrigerated tuna.

To investigate the reducing ability of met‐myoglobin (metMb) reductase, browned tuna meat was immersed with/without metMb reductase (0.103 unit/mL) for 120 min at 4°C. The metMb reductase activity of browned tuna meat significantly increased, whereas no significant change was observed in the control sample for the first 10 min of immersion (P > 0.05). After comparing the changes in metMb reductase activity, metMb content, Hunter’s ‘a’ and color, it was observed that the reductase did recover the bright red color. To further confirm the role of metMb reductase in color stability, fresh tuna meat was first immersed in metMb reductase (0.503 unit/mL) or a mixture of reductase and nicotinamide adenine dinucleotide (NADH) (1 mM) solutions at 5°C for 10 min, and then stored at 4°C and relative humidity 100%. No significant changes in metMb accumulation and in metMb reductase activity were observed between samples treated with enzyme alone and enzyme + NADPH (P > 0.05). Changes in metMb reductase activity, metMb, Hunter’s ‘a’ value and color for both the control and treated samples suggested that immersion in metMb reductase could extend the color stability of tuna meat, and that the enzymatic reduction of metMb by its reductase occurred during refrigerated storage.

SUMMARY : Metmyoglobin (Met‐Mb) reductase from blue‐fin tuna ordinary muscle was purified to electrophoretical homogeneity by ammonium sulfate fractionation, ion exchange and organomercurial agarose affinity chromatography. The molecular weight estimated by SDS‐PAGE was 100 kDa. The optimal pH and temperature for the reduction of met‐Mb were 7.3 and 25°C, respectively. This enzyme was very stable at pH 7.0–7.3 and 4°C–15°C. The purified enzyme was strongly activated by K+, moderately activated by Na+ and Mn2+, but not affected by Ni2+. It was moderately inhibited by Li+, NH4+, Mg2+, and Co2+. The Vmax for the reduction of met‐Mb and NADH were 0.32 and 0.43 mM/min/mg, while Km were 2.3 × 10−5 and 24.4 × 10−5 M, respectively.