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; longjaw mudsucker), the cytosolic form of malate dehydrogenase (cMDH) is encoded by two gene loci. One locus encodes a cMDH isozyme having a higher 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 muscle 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 stability 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 caused changes in the ratio of the two isoforms. The rate of change in isozyme ratio, as indexed by shifts in thermal stability of cMDH, was higher for the fish transferred from 10 degrees C to 30 degrees C than for fish undergoing the reciprocal transfer. Differential expression of cMDH isozymes favors conservation of K-m values at physiological temperatures and may contribute to the high degree of eurythermality characteristic of this species.