Thermal behavior of a lactate dehydrogenase purified from the heterothermic and sympatric vertebrate species brook lamprey (Lampetra planeri), tench (Tenca tenca), smooth (Triturus vulgaris) and alpine newt (Triturus alpestris)

1. 1. The A₄ lactate dehydrogenase isozyme was purified to homogeneity from the tissues of Brook lamprey (Lampetra planeri) tench (Tenca tenca), smooth newt (Triturus vulgaris) and alpine newt (T. alpestris). 2. 2. These four species share their geographical distribution in the same freshwater habitats, often live together in the same station and two of them are congeneric. Steady-state kinetic investigations have shown that: 3. 3. K_m (apparent) for pyruvate vs. temperature and (apparent) product K_i (Pyruvate) and K_i (Lactate) are fairly similar among species; 4. 4. k_cat/K_m decreases with temperature in the case of the newts but increases in the case of both lamprey and tench; 5. 5. Thermostability does not correlate to preferred ambient temperature and, in particular, tench LDH starts being inactivated up to 65°C. 6. 6. Thermostability does not correlate with activation energy either; 7. 7. No clear relationships can be demonstrated either between activation energy and conformational transitions in the molecule (these latter indicated by breaks in the Arrhenius plots) nor between activation energy and molecular flexibility, investigated by melting experiments.