Evolution of a nervous system specific lactate dehydrogenase isozyme in fish

L‐lactate dehydrogenase (LDH) exists as tetrameric isozymes in all vertebrates. The polypeptide subunits are encoded in at least two genetic loci, A and B. A third LDH locus, the E locus, is present only in fish. The distinctive E₄ isozyme encoded by this locus is synthesized in the cells of the teleost nervous tissue, particularly in the photoreceptor cells. Electrophoretic and immunochemical procedures were employed to determine which orders of fish have representatives which contain this nervous system specific LDH isozyme. In addition to the 50 species of teleostean and non‐teleostean fish examined, a compilation was made of other species reported to contain this enzyme. The LDH E gene function was not detected in Agnathans, Chondrichthyes, Dipnoans, Chondrosteans, and Holosteans. The LDH E gene appears to be restricted to the teleosts. However, some orders of teleosts, e.g., the Cypriniformes, contain many families which lack the E₄ isozyme. The LDH E gene appears to have arisen from a duplication of the LDH B gene near the time of the adaptive radiation of the teleosts. The immunochemical analyses of the LDH isozymes from these fish indicate that the LDH A and B loci of non‐teleostean fish have not diverged to the extent as these same loci in teleosts.