TEMPERATURE-DEPENDENT CHANGES IN EXPRESSION OF THERMOSTABLE AND THERMOLABILE ISOZYMES OF CYTOSOLIC MALATE-DEHYDROGENASE IN THE EURYTHERMAL GOBY FISH GILLICHTHYS MIRABILIS

In the eurythermal teleost Gillichthys mirabilis (family Gobiidae; lon gjaw mudsucker), the cytosolic form of malate dehydrogenase (cMDH) is encoded by two gene loci. One locus encodes a cMDH isozyme having a hi gher thermal stability and a lower apparent Michaelis-Menten constant (K-m) of cofactor (NADH;) than the isozyme encoded by the second locus . The ratio of thermostable to thermolabile cMDHs in white skeletal mu scle varied with exposure temperature. Winter-acclimatized fish had a higher percentage of thermolabile cMDH than summer-collected specimens , as demonstrated by electrophoretic analysis, enzyme thermal stabilit y measurements, and K-m of NADH determinations. Laboratory acclimation of fish to 10 degrees C and 30 degrees C for a period of 6 wk also ca used changes in the ratio of the two isoforms. The rate of change in i sozyme ratio, as indexed by shifts in thermal stability of cMDH, was h igher for the fish transferred from 10 degrees C to 30 degrees C than for fish undergoing the reciprocal transfer. Differential expression o f cMDH isozymes favors conservation of K-m values at physiological tem peratures and may contribute to the high degree of eurythermality char acteristic of this species.