The effects of neutral salts, such as KCl, NaCl and LiCl, in the thermal inactivations of myofibrillar Ca-ATPase of white croaker and carp have been studied. The myofibrils were kept at a fixed temperature in the range of 25-38°C with various concentrations (0.035-2.0M) of KCl or NaCl and the first order rate constants (k_D) for inactivation of Ca-ATPase were estimated.
Upon increasing the concentration (M) of neutral salt in the medium, the increase in k_D proceeded in three first order steps: a very slow rate of increase from 0.035 to 0.30M, a rapid rate of increase from 0.30 to 1.1M (for white croaker) or to 1.5M (for carp), and remaining constant at a maximum above 1.1M (for white croaker) or above 1.5M (for carp). The linear rate plots of log k_D vs. salt concentration (0.3-1.0M) showed that white croaker myofibrils were 2.5 times more susceptible to neutral salt than carp myofibrils. The following relative k_D values for inactivation of Ca-ATPase by neutral salts were found: LiCl>NaCl>KCl.
The effect of neutral salts on the k_D for inactivation of its Ca-ATPase was identical for myosin B and for myofibrils from the same fish species.