Endogenous proteinases in true sardine (Sardinops melanostictus)

Characterisation of the autolytic profile of true sardine (Sardinops melanostictus) indicated the involvement of heat-activated proteinases, active at both acidic and alkaline pH values. Autolytic activity decreased with increasing NaCl concentration (0–30%). When crude proteolytic activity in true sardine was studied, two activity peaks were observed, at pH 3.5 and 9.0. Crude proteinase extracts exhibited the highest activity at 55 °C and 60 °C when assayed at pH 3.5 and 9.0, respectively. The pH 3.5 peak activity was effectively inhibited by pepstatin A, while the pH 9.0 peak activity was mostly inhibited by soybean trypsin inhibitor, PMSF and TLCK, suggesting that the various proteinases were present in true sardine. The enzymes were stable for up to 8 h at 55 °C. The activities were also stable at a pH range of 2.0–4.0 and still retained high activity toward hemoglobin after incubation at pH 3.5 for 8 h. Activities of the crude extract continuously decreased as NaCl concentration increased. ATP showed no effect on enzyme activity, while CaCl2 and MgCl2 activated the proteinase activity. The results implied that pepsin is a predominant enzyme responsible for autolysis in true sardine during fish sauce fermentation.