Effect of inorganic polyphosphate on thermal denaturation of carp myofibrillar protein at low ionic strength

The increase in pH and ionic strength of carp myofibrils suspension was induced by the addition of inorganic polyphosphates, such as sodium pyrophosphate, sodium tripolyphosphate and SODIUM POLYPHOSPHATE (Food additive). Thus, the effects of increase in pH and ionic strength were separately examined by measuring the change in the first order rate constant (kD) for thermal inactivation of myofibrillar Ca-ATPase. To the myofibrils suspension containing various concentrations of inorganic polyphosphate, KCl was added to fix its ionic strength. The kD values of these myofibrils were found to be minimum at pH 7.0-7.5. As the myofibrillar protein concentration was increased beyond 20 mg/ml, the pH of myofibrils suspension became neutral with 0.3% inorganic polyphosphate, regardless of whether its original pH was at 5.9 or 6.9. The sum of ionic strength given by KCl and inorganic polyphosphate was increased from 0.03 to around 0.1, virtually no change was found in the kD values, but with the increase from 0.1 to over 0.2, a gradual increase in the kD was observed. These results revealed that when inorganic polyphosphate was mixed with myofibrils suspension to make its ionic strength fixed at nearly 0.1 and to make its pH neutral, the preventive effect of inorganic polyphosphate against thermal denaturation of myofibrillar protein was maximally exhibited.