Promoting Effect of Urea and pH on Thermal Denaturation of Myofibrillar Protein from Shark KojiHASIMOTO,*1 Takafumi NISHIKIORI,*2 and Ken-ichi ARAI*3

The myofibril suspension (in 0.16M KCI-40 mm Tris-HCI , pH 7.5 or 0.16tit KCI-40 mm Trismaleate, pH 5.8)of blue shark and tiger shark was kept at a fixed temperature in the range of 20-45°C with various concentrations of urea and measured for its Ca-ATPase activity with a lapse of tme. In the presence of urea, myofibrillar Ca-ATPase of blue shark inactivated in two first order reactions with an early fast rate (kDt) and subsequently with a slow rate (kDs), whereas the inac tivation of myofibrillar Ca-ATPase of tiger shark fitted by a single exponential with a rate of kD. Plotting the logarithm of rate constant against the concentration (M) of urea, a linear relation was thus obtained. The slope(D)of the graph of logarithm of rate constant versus urea (M) indicated that myofibrillar protein of blue shark was more susceptible to urea than that of tiger shark. By introducing an increase in the rate constant as a measure of accelerating effect on de naturation of myofibrillar protein, multiplying effect by the presence of urea (0.3M), the fall in pH (from 7.5 to 5.8) and an increase in temperature was observed for both shark species.