Lactate dehydrogenase isozymes of the flatfish, Pleuronectiformes: Kinetic, molecular and immunochemical analysis

At least two genes are responsible for lactate dehydrogenase (LDH) synthesis in the tissues of eighteen species of flatfish (Pleuronectiformes). The relative activity of these genes is markedly skewed with LDH‐A4 being present in much larger amounts and in a wider variety of tissues than LDH‐B4. Certain flatfish exhibit a single LDH‐A4 while others have five forms of this tetramer. Molecular hybridization experiments demonstrate that these five tetramers have two distinguishable subunits. These subunits were purified and subjected to amino acid analysis, peptide mapping, kinetic and immunochemical analysis and shown to be very similar, perhaps identical, in primary structure. Treatment with reducing reagents failed to interconvert them. LDH‐B4 was partially purified and shown to be kinetically and immunochemically distinct from LDH‐A4. As is true for many other fish, additional LDH isozymes are obseved in eye and brian tissues. These results show that the flatfish genetic information for LDH synthesis is similar to that of other fish, and flatfish LDH A and B subunits are homologous to those of other vertebrates.