Thermal and urea labilities of actomyosin and myosin Ca²⁺-ATPase from several fish species have been investigated at constant pH, KCI and tris-maleate buffer concentration.
A comparison of the first order denaturation rate constants (K_D) of the two proteins at 30°C and 35°C revealed that both actomyosin and myosin of tilapia are most stable, followed by the skipjack, yellowfin and bigeye tunas, carp, star-spotted shark, yellowtail, flatfish and cod, in decreasing order of stability. But, both proteins from tilapia are remarkably unstable in comparison with those of rabbit.
The order of stability of Ca²⁺-ATPase, as established from the K_D values for urea denaturation, is the same as the above excepting the star-spotted shark. The actomyosin Ca²⁺-ATPase of this species is more resistant to thermal denaturation than those of carp and the tunas, whereas its urea lability is less than that of carp and comparable to those of the tunas.
The mechanism of thermal and urea denaturation and the molecular adaptation of both muscle proteins in fishes has been discussed.