Functional significance of isoenzymes in thermal acclimatization. Acetylcholinesterase from trout brain.

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 degrees C; a single ;cold' variant appears after acclimatization to 2 degrees C. Both forms are present in fish after acclimatization to an intermediate temperature. 3. The K(m) 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 K(m)-temperature relationship is adaptive, and that the critical process during thermal acclimatization, in cases where enzymes show sharp changes in K(m) 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.