Thermal denaturation profiles of catfish and tilapia myofibrils as affected by pH for heating

Thermal denaturation profiles of catfish myosin when heated as myofibrils (Mf) were compared with those of tilapia. The Ca²⁺-ATPase inactivation rate of catfish myofibrils was the same as that of tilapia myofibrils. The conclusion was the same with isolated myosin. Catfish Mf was clearly distinguished from tilapia Mf in terms of subfragment-1 (S-1) and rod denaturation. Quick denaturation of rod relative to S-1 was characteristic of catfish Mf, while slower denaturation of rod relative to S-1 was the pattern for tilapia Mf. These patterns were greatly affected by the pH for heating. Rod denaturation was accelerated with increasing pH for heating and oppositely suppressed by lowering the pH, for both Mf. Tilapia Mf showed a S-1 and rod denaturation pattern similar to that for catfish Mf, but at 1 pH unit higher; for example, the pattern of catfish Mf at pH 7.5 was similar to that for tilapia Mf at pH 8.5. Less rigid filament structure of catfish Mf than tilapia Mf was demonstrated by studying chymotryptic digestion at various pH values. Accordingly, the difference in the S-1/rod denaturation patterns between the two fish species can be explained by the different rigidity of their myosin filaments.