Gene duplication in salmonid fishes: Evidence for duplicated but catalytically equivalent A4 lactate dehydrogenases

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Abstract
Skeletal muscle tissues from many species of salmonid fish are known to exhibit a set of three to five isozymes for A4-type lactate dehydrogenase (l-lactate: NAD oxidoreductase, E.C. 1.1.1.27), but the genetic basis for this isozyme system has not previously been assessed. This isozyme system was purified to homogeneity from salmon (Oncorhynchus tshawytscha) and shown to be composed of two polypeptides, Aα and Aβ, in binomial tetrameric combinations. Amino acid analysis revealed that Aα and Aβ are closely related but genetically distinct proteins, and thus are coded for by recently duplicated structural loci. Catalytic studies on the purified intact salmon isozyme system and on the isozymically pure Aα4 and Aβ4 homotetramers from brown trout (Salmo trutta) revealed no significant differences in catalytic properties among these enzymes, suggesting equivalent catalytic function of Aα and Aβ. These results, in combination with studies on polypeptide and isozyme ratios, suggest that one of the duplicated loci in salmon may be drifting toward a nonfunctional state by accumulation of mutations in regulatory DNA rather than in the structural gene itself.