Effect of pH on sedimentation of cod (Gadus morhua) muscle membranes

A new process solubilizes muscle proteins, making it possible to separate membranes with their oxidizable phospholipids, but it requires a high centrifugal force to separate the membranes. In this study we show that isolated cod muscle membranes could be readily sedimented at a low centrifugal g force for 15 min at pH 5 or below but not at pH 6 or higher. Isoelectric aggregation is likely to be responsible for the sedimentation of isolated membranes at these low pH values. Various consecutive pH treatments on isolated membranes showed that certain proteins were resolubilized or resuspended at a final pH of 10.5 if they had been previously exposed to pH 3, 4, or 5. pH adjustment to 10.5 before adjustment to a pH of 5 or lower inhibited the sedimentation of isolated membranes. Isolated membranes added to the supernatant of homogenized muscle solubilized at pH 3 and centrifuged at 10000 x g for 30 min also sedimented at 4000 x g for 15 min. This was in contrast to the membranes naturally present in the solubilized homogenate. This suggests that factors other than viscosity were involved in the lack of efficient sedimentation of membranes from solubilized muscle tissue.