Temperature‐adaptive differences between the M4 lactate dehydrogenases of stenothermal and eurythermal sciaenid fishes

The effects of temperature on the apparent Michaelis‐Menten constant (Km) of pyruvate were determined for M4‐lactate dehydrogenases (LDHs) of 15 species of the teleost family Sciaenidae from habitats differing in average temperature and temperature range. The central question addressed was whether adaptive difference in the effect of temperature on an important kinetic parameter, Km, reflect selection based on the range of temperatures experienced by the species, the highest temperature encountered by the species, or both of these factors. For the M4‐LDHs of all 15 species, the Km of pyruvate was conserved between 0.1 and 0.4 mM pyruvate over the species' physiological temperature range. However, large increases in Km of pyruvate commonly were found at temperatures a few degrees C above the species' upper habitat temperature. Distinctions were found between the responses of the enzymes of eurythermal and stenothermal sciaenids. For the most eurythermal species, e.g., Sciaenops ocellatus, Micropogonias undulatus, and Roncador stearnsii, Km of pyruvate varied only about twofold between 10 and 30°C, the temperatures which encompass most of the habitat ranges of these species. For the most stenothermal species, e.g., Cynoscion striatus and Micropogonias furnieri, with habitat temperature ranges of 14–18°C, sharp increases in Km occurred at temperatures above 20°C. We hypothesize that thermal stability of Km values is more strongly selected by maximal habitat temperature than by the amount of variation in habitat temperature, and that adaptation to high temperatures may pre‐adapt enzymes for eurythermal function.