Isolation, characterisation and stability of myoglobin from Eastern little tuna (Euthynnus affinis) dark muscle

This study aimed to purify and characterise the myoglobin from the dark muscle of Eastern little tuna (Euthynnus affinis). Myoglobin purified by ammonium sulphate precipitation (65–100% saturation), followed by Sephadex G-75 chromatography had a molecular weight of 15,680 Da. The isoelectric point (pI) of both oxymyoglobin (OxyMb) and metmyoglobin (MetMb) was estimated to be 5.25, as determined by zeta potential analysis. Transition temperatures were 61 and 60 °C, for OxyMb and MetMb, respectively. The colour values (L∗, a∗ and b∗) and absorption spectra of the myoglobin solutions differed significantly, depending upon the form of myoglobin. OxyMb and MetMb had the Soret bands at 413 and 407 nm, respectively. The pH and thermal stability of myoglobin were tested under a pH range of 3–11 and a temperature range of 4–70 °C, respectively. Loss of haem–globin complex and autoxidation were dominant at pH 3, as evidenced by the disappearance of the Soret band and the formation of MetMb. Heating at temperature above 60 °C had a great impact on myoglobin denaturation. With increasing temperature and incubation time, OxyMb was susceptible to oxidation and conformational changes, whilst MetMb tended to be more stable. Thus, the form of myoglobin governed its properties and stability.