Characterisation of myoglobin from sardine (Sardinella gibbosa) dark muscle

Myoglobin from the dark muscle of sardine (Sardinella gibbosa) with the molecular weight of 15.3 kDa was isolated and characterised. The different myoglobin derivatives exhibited varying thermal unfolding characteristics. Deoxymyoglobin showed a single distinct endothermic peak at 74.5 °C, whereas two transition temperatures were noticeable for oxymyoglobin (64.5 and 78.4 °C) and metmyoglobin (59.0 and 76.0 °C). The spectrum of deoxymyoglobin and oxymyoglobin had absorption bands at 739, 630, 575, 500 and 405 nm, while the disappearance of the peak at 575 nm was found in the spectrum of metmyoglobin. The soret peak of all derivatives was noticeable at 405 nm. The autoxidation of myoglobin became greater at very acidic or alkaline conditions as evidenced by the formation of metmyoglobin, the changes in tryptophan fluorescence intensity as well as the disappearance of soret absorption. The higher temperature, particularly above 40 °C, and the longer incubation time induced the higher metmyoglobin formation as well as the conformational changes.