Stability of sardine myofibrils during storage at various pH values.

Abstract

The effect of pH on the denaturation profile of sardine ordinary and dark muscle myofibrils under various storage conditions was investigated by measuring Mg2+-ATPase activity and solubili-ty. Storage at O°C for 2 days gave rise to two activity maxima both in the presence and absence of Ca2+, one at pH azound 7.5 and another at pH 9.5, and the highest Ca-sensitivity at pH 7.5regardless of muscle type. After 2 days of storage at-20°C, the remaining activities were maxi-mal at pH around 9.5 either in the presence or absence of Ca2+, while extremely low at pH between 5.5 and 7.5 for both myofibrils. When myofibrils were incubated at 35°C for 30min, Mg2+-ATPase activities in the presence and absence of Ca2+were both increased with the maximum values at pH 7 for ordinary muscle and at pH 7.5 for dark muscle myofibrils. Ca-sensitivity of ordinary musclemyofibrils almost disappeared while that of dark muscle to some extent remained, especially in alkaline pH range. There was no marked change in solubility and SDS-PAGE pattern of soluble protein fraction throughout pH examined with both myofibrils stored either at 0 or-20°C. The solubility of ordinary muscle myofibrils decreased below 40%at any pH after incubation at 35°C whereas SDS-PAGE pattern showed no myosin heavy chain in the soluble protein fraction. The solubility of dark muscle myofibrils also decreased considerably, especially in an alkaline pH range. This well corresponded to the SDS-PAGE pattern of the soluble protein fraction where less amount of myosin heavy chain was observed at pH above 7.5.