Pressure-Temperature Interactions on M4-Lactate Dehydrogenases From Hydrothermal Vent Fishes: Evidence for Adaptation to Elevated Temperatures by the Zoarcid Thermarces andersoni, but not by the Bythitid, Bythites hollisi.

Lactate dehydrogenases (LDH; M4 isozyme) were purified from skeletal muscle taken from two fishes endemic to hydrothermal vents, Thermarces andersoni (Zoarcidae; 13: °N, East Pacific Rise, depth [sim] 2600 m) and Bythites hollisi (Bythitidae; Galapagos Spreading Center, depth [sim] 2500 m), and from the cosmopolitan deep-sea rattail Coryphaenoides armatus (Macrouridae; depth of occurrence to [sim] 5000 m). The effects of pressure and temperature on the apparent Michaelis-Menten constant (Km) of cofactor (NADH) were measured to compare sensitivities to temperature, at in situ pressures, of enzymes from hydrothermal vent fishes and from a species adapted to cold, stable deep-sea temperatures. At 5: °C, the Km of NADH of the M4-LDHs of the three species varied only slightly between measurement pressures of 1 and 340 atmospheres (atm), in agreement with earlier studies of M4-LDHs of deep-sea fishes. At higher measurement temperatures, marked differences were found among the enzymes. For the M4-LDHs of C. armatus and B. hollisi, increases in temperature (10 to 20: °C), at in situ pressures, sharply increased the Km of NADH to values higher than those predicted to be physiologically optimal. The M4-LDH of T. andersoni exhibited only minimal perturbation by elevated temperature under in situ pressures. The different temperature-pressure responses of these LDHs suggest that enzymes of deep-sea fishes not endemic to hydrothermal vents are not adapted for function at the higher temperatures found at vent sites, and that T. andersoni is better adapted than B. hollisi for sustained exposure to warm vent waters. The importance of adaptation to warm temperatures in the colonization of vent habitats is discussed.