1. The effects of acclimatization temperature on the catalytic properties of acetylcholinesterase from rainbow-trout brain were examined. 2. Trout brain acetylcholinesterase occurs in two distinct forms. A single ‘warm’ variant of the enzyme is present after acclimatization to 17°C; a single ‘cold’ variant appears after acclimatization to 2°C. Both forms are present in fish after acclimatization to an intermediate temperature. 3. The Km values of the enzyme variants for acetylcholine are temperature-dependent, the lowest values coinciding with the acclimatization temperature at which each enzyme was induced. 4. It is concluded that the Km–temperature relationship is adaptive, and that the critical process during thermal acclimatization, in cases where enzymes show sharp changes in Km with temperature, is the synthesis of a new enzyme variant that is better suited for catalysis and control of catalysis under the conditions of the acclimatized state.

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