A Lactate Dehydrogenase Specific to the Liver of Gadoid Fish

Abstract

Comparative studies on tissue extracts of haddock, cod, and related species in the suborder Gadoidei indicated the presence of a lactate dehydrogenase specific to liver tissue. Genetic and evolutionary variation of the lactate dehydrogenase isozymes in these fishes indicate that the liver isozyme is encoded at a structural gene locus genetically distinct from the loci encoding the heart and muscle isozymes. The properties of purified liver, heart, and muscle lactate dehydrogenases from haddock were compared to investigate their relationship. The haddock liver enzyme is like other lactate dehydrogenases in having a molecular weight of 140,000 with a subunit size of 35,000. The liver enzyme is distinguished from the heart and muscle isozymes by its greater resistance to heat inactivation. The gadoid heart and muscle isozymes do not hybridize with each other in vivo or in vitro, but the liver enzyme can hybridize with both in vitro, although not in a random fashion. The catalytic properties of the haddock liver and heart isozymes are very similar; they are distinct from the muscle isozyme in having a greater affinity for both lactate and pyruvate and in being inhibited at high pyruvate concentrations. The liver enzyme showed no unusual substrate specificities. Ironically, the liver enzyme can be mistaken for the muscle enzyme since both have nearly identical electrophoretic mobilities. The catalytic properties of the haddock heart and liver enzymes suggest that these two may be homologous and that both might be homologous to the heart-type lactate dehydrogenase of the higher vertebrates. The immunological crossreactivity of these two with each other and with chicken heart lactate dehydrogenase provides confirmation for this hypothesis. The immunological data suggest a considerable degree of sequence homology between the heart and liver lactate dehydrogenases of gadoid fishes. From evolutionary considerations, it appears that the divergence of the heart and liver loci from an ancestor heart-type locus is a relatively recent event, perhaps arising concurrently with the evolution of the gadoid fish into a distinct lineage.