A monophyletic origin of heart-predominant lactate dehydrogenase (LDH) isozymes of gnathostome vertebrates: evidence from the cDNA sequence of the spiny dogfish (Squalus acanthias) LDH-B.

Duplication of a single lactate dehydrogenase locus early in vertebrate evolution has been proposed to have given rise to Ldh-A and Ldh-B, the encoded isozymes of which predominate in skeletal and heart muscle, respectively. This view has been challenged recently by phylogenetic analyses of LDH sequences. One question that has been raised is whether the heart-predominant isozyme (LDH-B) of cartilaginous fishes is orthologous to that of bony fishes and their derivatives. To address this issue, we determined the complementary DNA sequence of the LDH-B of the chondrichthyan Squalus acanthias. Phylogenetic analysis of this and other LDH isozyme sequences provided strong support for a single origin of LDH-Bs prior to the divergence of cartilaginous and bony fishes.