QUALITATIVE AND QUANTITATIVE STRATEGIES OF THERMAL ADAPTATION OF GRASS CARP (CTENOPHARYNGODON-IDELLA) CYTOPLASMIC MALATE-DEHYDROGENASES

Malate dehydrogenase isozymes of grass carp, Ctenopharyngodon idella, were identified by mitochondrial preparation and thermal denaturation. The structural and kinetic characteristics of chromatographically sep arated thermostable and thermolabile cMDHs were different in (1) half- life at 42 degrees C, 10 min and 24 h, respectively, (2) optimal subst rate, oxaloacetate and malate, concentrations, and (3) the apparent Mi chaelis-Menten constants of NADH and oxaloacetate. Total MDH activity in white muscle of 11 degrees C-acclimated fish was about twice that o f the 30 degrees C-acclimated group. In addition, the ratio of the the rmostable to thermolabile cMDH activity in white muscle of 30 degrees C-acclimated fish was significantly higher than that of 11 degrees C-a cclimated fish. These results suggest that temperature acclimation can induce temperature compensation in MDH activity and differential expr ession of thermostable and thermolabile cMDH isozymes in freshwater fi sh.