SEASONAL CHANGES IN THERMODYNAMIC ACTIVATION PARAMETERS OF MYOSIN Ca2+-ATPase FROM GRASS CARP AND SILVER CARP

Eurythermal temperate freshwater fish such as grass carp, silver carp and common carp often experience large seasonal changes in body temperature.These show a high capacity in temperature adaptation which enables them to maintain locomotory performance throughout the year.The mechanisms underlying such changes in swimming performance have been shown to involve changes in the properties of muscle protein such as the thermal stability of myofibrillar ATPase.Myosin amounts to about over 50% of myofibrillar protein in fish skeletal muscles, and its properties mainly decide properties of the myofibrillar protein.Thus, the processing adaptabilities of fish muscular proteins are virtually influenced by the properties of myosin.In this paper, fast skeletal muscles were dissected from the dorsal epaxial myotomes of grass carp (Ctenopharyngodon idellus Valenciennes) and silver carp (Hypophthalmichthys molitrix Valenciennes) in spring (April), summer (August), autumn (November) and winter (January) , taking care to avoiding contamination with slow muscle, immediately used as preparation of myosins.The purity of myosin preparations was examined by SDS-PAGE.The results showed that themyosins did not contain any appreciable amount of actin, demonstrating their high purity1Each of the purified myosin preparations contained heavy chain having a molecular mass of about 200 kDa and three kinds of light chains of about 20 kDa including alkali light chains A1 and A2, and 5, 5c-dithio-bis-2-nitrobenzoic acid (DTNB) light chain.Thermodynamic activation parameters of myosin Ca2+-ATPase, activation energy (Ea), enthalpy of act ivation , entropy of activation and free energy of activation ,were determined in order to illustrate the relationship between thermal stabilities of myosin and seasonal temperature changes at thermal dynamic theory of chemical reaction.The results showed that three types of thermodynamic activation parameter, activation energy (Ea), enthalpy of activation and entropy of activation were 14319kJ/mol, 14113kJ/mol and 40916 e1u1, respectively, for myosin from the summer grass carp, and 11711kJ/mol, 11416kJ/mol and 32416 e1u1, respectively, for myosin from the winter grass carp, indicating that these three parameters increased with the habitat environment temperatures increasing.Whereas, myosins from the spring and autumn grass carp showed the values comparable to those from the summer and winter grass carp, respectively, and the former was slightly lower than those of the summer fish and the latter was slightly higher than those of the winter fish.However, no significant changes in free energy of activation were observed.On the other hand, changes in these thermodynamic activation parameters for myosins from silver carp in different seasons were similar to those for myosins from grass carp.These results demonstrated that thermal stabilities of myosins from freshwater fish in different seasons were different, namely the thermal stability of myosin from the summer fish was significantly higher than that from the winter fish,whereas myosins from the spring and autumn fish had thermal stabilities similar to those from the summer and winter fish, respect ively.In addition, a good correlation between H琣nd S?was found, suggesting the existence of a significant compensatory effect in these two parameters,which is necessary for freshwater fish to adapt to the changes in environment temperature.