First order rate constants (KD) for inactivation of myofibrillar Ca-ATPase from several fish species and rabbit were measured at various pHs (5.1-7.6) and at various temperatures (1-50°C). The ARRHENIUS plots for most KD thus obtained were found to be linear. Moreover, as pH falls, whole linear plots shifted to lower temperature range, and the slope became more gentle. In fact, the plots at pH 5.8 and pH 5.4 for myofibrils of skipjack tuna (warm water fish) nearly coincided with those at pH 7.6 for sardine (temperate water fish), and for atka mackerel (cold water fish). Thermodynamic activation parameters for the inactivation decreased with pH fall, though the proportions of ΔH≠ as well as ΔS≠ varied considerably and C varied somewhat.
In contrast, at lower pH than 6.1 for temperate and cold water fishes, 5.4 for warm water fish, and 5.2 for rabbit myofibrils, ARRHENIUS plots consisted of two linear regions with a break in the slope at around 16-23°C. Since such a plot over lower temperature range had a rather gentle slope, a reduction of denaturation rate was accomplished more effectively by neutralization of acid than by lowering temperature.
From these results, a possible rate and degree of denaturation of myofibrillar proteins was discussed in relation to post-morten change in muscular pH and variation of storage temperature of these fishes.