Evolutionary temperature adaptation and the calcium regulation of fish actomyosin ATPases

Actomyosins have been isolated from the white skeletal muscles of fish inhabiting thermal environments with characteristically low seasonal temperature variations for example, Antarctica, arctic rivers and the equatorial oceans. Calcium regulation of actomyosin ATPase was correlated with environmental temperature. Increased assay temperature resulted in a progressive loss of calcium sensitivity. This occurred at lower temperatures in the more cold adapted species. In general cold adapted ATPases have lower activation enthalpies (ΔH‡) and higher activities at low temperatures than their tropical counterparts. In contrast to environmental temperature acclimation (Johnston, 1979) calcium regulatory proteins of stenothermal species were not directly involved in the temperature adaptation of actomyosins. Similar relationships were obtained between environmental temperature and thermodynamic activation parameters for both natural and desensitized actomyosins from which tropomyosin and troponins had been removed. Instead, evidence is presented that the mechanism of temperature adaptation over evolutionary time periods involves selective modifications in the structure of both the heavy and light chains of myosin.