The effects of temperature upon acetylcholinesterase (AChE) from the nervous system of fish were studied to determine if such compensatory phenomena as thermal accommodation, thermal acclimation and evolutionary adaptation to temperature as displayed by this physiological system could be observed and interpreted at the level of enzyme function. At probable physiological substrate concentrations the rate of acetylcholine (ACh) hydrolysis by AChE from rainbow trout (Salmo gairdnerii) and electric eel remains relatively unaffected by assay temperature over the temperature ranges normally experienced by these animals. Plots of Km versus temperature for these enzymes yield U shaped curves with minimum Km values occurring at temperatures close to the minimum habitat temperature. It is proposed that thermal accommodation of reaction rate is achieved throughout the habitat temperature range by temperature directed changes in enzyme-substrate affinity. Thermal acclimation in rainbow trout, and probably in speckled trout (Salvelinus fontinalis) and lake trout (Salvelinus namaychus) is accompanied by alterations in the relative proportions of two electrophoretically distinct AChE variants displaying different and adaptive Km-temperature relationships. Since the minimum Km values and energies of activation of the two rainbow trout enzymes are similar, and the specific activities of the enzymes are essentially identical following acclimation of fish to 2° and 17°C, it is suggested that rate compensation of AChE activity may not occur at different acclimation temperatures. However, the possibility remains that changes in such factors as pH, ionic environment and membrane lipids which accompany the acclimation process may act to stabilize reaction rates. Comparisons of AChE enzymes from rainbow trout, electric eel and the Antarctic fish Trematomus borchgrevinki indicate that the evolutionary adaptation of AChE function in species inhabiting different thermal environments is based upon selection for a Km-temperature relationship that will allow thermal accommodation of reaction rate over the temperature range normally encountered. Shifts in the Km-temperature relationship during speciation are interpreted in terms of changes in enzyme conformation following the accumulation of amino acid substitutions. Possible mechanisms by which two AChE enzymes could be incorporated into the trout central nervous system were considered and a hypothesis involving hybridization between fish populations was tested with trout inter-species crosses. It was observed that hybrids formed between speckled and lake trout contained a greater number of electrophoretically distinct AChE variants than did either parent and further, the presence of similar thermally controlled AChE complexes in rainbow, speckled and lake trout indicated that the original incorporation of multiple AChE enzymes into the rainbow trout probably occurred prior to the evolutionary divergence of these three species. It is concluded from this study that changes in enzyme-substrate affinity with temperature, and the temperature directed production of enzyme variants displaying adaptive Km-temperature relationships, are both important mechanisms for controlling catalytic activity in an enzyme system which functions over a wide range of temperatures. / Science, Faculty of / Zoology, Department of / Graduate